Disable pytest cacheprovider plugin in CI

The pytest "cacheprovider" plugin produces a .cache/v/cache/lastfailed
file, which holds a Python dictionary structure with failed tests.
However, on Ubuntu 16.04 (Xenial) the file is created even though the
test passed and the file contains just an empty dictionary ("{}").

Given that we are not interested in this feature, disabling the
"cacheprovider" plugin globally and removing per-test removals of the
.cache directory seems like the best course of action.

(cherry picked from commit e1c30341)
77 jobs for !4964 with mnowak/disable-pytest-cacheprovider-plugin-v9_16 in 24 minutes and 23 seconds (queued for 8 seconds)
latest detached
Status Job ID Name Coverage
  Precheck
passed #1678369
amd64 linux
autoreconf

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passed #1678371
amd64 linux
clang-format

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passed #1678372
amd64 linux
coccinelle

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amd64 linux
danger

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misc

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  Build
passed #1678386
amd64 linux
clang:asan

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clang:buster:amd64

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amd64 libvirt
clang:freebsd11:amd64

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passed #1678394
amd64 libvirt
clang:freebsd12:amd64

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passed #1678395
amd64 libvirt
clang:openbsd:amd64

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passed #1678388
amd64 linux
clang:tsan

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passed #1678374
amd64 linux
gcc:alpine3.13:amd64

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passed #1678385
amd64 linux
gcc:asan

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amd64 linux
gcc:buster:amd64

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passed #1678375
amd64 linux
gcc:centos7:amd64

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passed #1678376
amd64 linux
gcc:centos8:amd64

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passed #1678384
amd64 linux
gcc:focal:amd64

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passed #1678389
amd64 linux
gcc:mutexatomics

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passed #1678380
amd64 linux
gcc:out-of-tree

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amd64 linux
gcc:sid:amd64

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passed #1678381
i386 linux
gcc:sid:i386

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passed #1678391
amd64 linux
gcc:softhsm2.4

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passed #1678392
amd64 linux
gcc:softhsm2.6

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passed #1678377
amd64 linux
gcc:stretch:amd64

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passed #1678387
amd64 linux
gcc:tsan

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passed #1678382
amd64 linux
gcc:tumbleweed:amd64

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passed #1678383
amd64 linux
gcc:xenial:amd64

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passed #1678397
amd64 windows
msvc:windows:amd64

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passed #1678396
amd64 linux
nolibtool:sid:amd64

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  Unit
passed #1678409
amd64 linux
unit:clang:asan

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passed #1678413
amd64 linux
unit:clang:buster:amd64

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amd64 libvirt
unit:clang:freebsd11:amd64

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passed #1678417
amd64 libvirt
unit:clang:freebsd12:amd64

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passed #1678411
amd64 linux
unit:clang:tsan

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passed #1678398
amd64 linux
unit:gcc:alpine3.13:amd64

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passed #1678408
amd64 linux
unit:gcc:asan

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passed #1678402
amd64 linux
unit:gcc:buster:amd64

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passed #1678399
amd64 linux
unit:gcc:centos7:amd64

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passed #1678400
amd64 linux
unit:gcc:centos8:amd64

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passed #1678407
amd64 linux
unit:gcc:focal:amd64

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passed #1678412
amd64 linux
unit:gcc:mutexatomics

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passed #1678403
amd64 linux
unit:gcc:sid:amd64

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passed #1678404
i386 linux
unit:gcc:sid:i386

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passed #1678414
amd64 linux
unit:gcc:softhsm2.4

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passed #1678415
amd64 linux
unit:gcc:softhsm2.6

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passed #1678401
amd64 linux
unit:gcc:stretch:amd64

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passed #1678410
amd64 linux
unit:gcc:tsan

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passed #1678405
amd64 linux
unit:gcc:tumbleweed:amd64

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passed #1678406
amd64 linux
unit:gcc:xenial:amd64

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passed #1678418
amd64 linux
unit:nolibtool:sid:amd64

00:00:58

 
  System
failed #1678430
amd64 linux
system:clang:asan

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passed #1678434
amd64 linux
system:clang:buster:amd64

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passed #1678437
amd64 libvirt
system:clang:freebsd11:amd64

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passed #1678438
amd64 libvirt
system:clang:freebsd12:amd64

00:10:30

passed #1678432
amd64 linux
system:clang:tsan

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passed #1678419
amd64 linux
system:gcc:alpine3.13:amd64

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passed #1678429
amd64 linux
system:gcc:asan

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passed #1678423
amd64 linux
system:gcc:buster:amd64

00:13:18

passed #1678420
amd64 linux
system:gcc:centos7:amd64

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passed #1678421
amd64 linux
system:gcc:centos8:amd64

00:12:12

failed #1678428
amd64 linux
system:gcc:focal:amd64

00:12:23

passed #1678433
amd64 linux
system:gcc:mutexatomics

00:12:10

passed #1678424
amd64 linux
system:gcc:sid:amd64

00:12:02

passed #1678425
i386 linux
system:gcc:sid:i386

00:10:37

passed #1678435
amd64 linux
system:gcc:softhsm2.4

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passed #1678436
amd64 linux
system:gcc:softhsm2.6

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passed #1678422
amd64 linux
system:gcc:stretch:amd64

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passed #1678431
amd64 linux
system:gcc:tsan

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passed #1678426
amd64 linux
system:gcc:tumbleweed:amd64

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failed #1678427
amd64 linux
system:gcc:xenial:amd64

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passed #1678439
amd64 linux
system:nolibtool:sid:amd64

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  Docs
passed #1678440
amd64 linux
docs

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  Postcheck
passed #1678444
amd64 linux
cppcheck

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passed #1678441
amd64 linux
flake8

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passed #1678445
amd64 linux
gcov

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75.0%
passed #1678442
amd64 linux
pylint

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passed #1678443
amd64 linux
scan-build

00:09:24

 
Name Stage Failure
failed
system:clang:asan System
No job log
failed
system:gcc:focal:amd64 System
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failed
system:gcc:xenial:amd64 System
No job log
failed
danger Precheck
No job log