Disable pytest cacheprovider plugin on Ubuntu Xenial

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 ("{}").

Disabling the "cacheprovider" plugin on Xenial and removing per-test
removals of the .cache directory seems like the best course of action.
80 jobs for !4958 with mnowak/xenial-disable-pytest-cacheprovider-plugin in 1 minute and 16 seconds (queued for 10 seconds)
latest detached
Status Job ID Name Coverage
  Precheck
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clang-format

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coccinelle

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misc

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autoreconf

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  Unit
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unit:gcc:stretch:amd64

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  System
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system:clang:asan

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system:clang:tsan

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  Docs
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  Postcheck
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cppcheck

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flake8

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gcov

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pylint

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