Enable Test Coverage Visualization for merge requests

This feature allows GitLab to visualize test coverage information in the
file diff view of merge requests.

This commit makes the gcov CI job depend on the following chain of jobs:

    gcc:buster:amd64 → unit:gcc:buster:amd64 → system:gcc:buster:amd64

The reason for running the last two jobs above sequentially rather than
in parallel is that both of them create *.gcda files (containing
coverage data) in the same locations.  While some way of merging these
files from different job artifact archives could probably be designed
with the help of additional tools, the simplest thing to do is not to
run unit test and system test jobs in parallel, carrying *.gcda files
over between jobs as gcov knows how to append coverage data to existing
*.gcda files.

Also note that test coverage will not be visualized if any of the jobs
in the above dependency chain fails (because the gcov job will not be
run).

(cherry picked from commit 2dabf328)
80 jobs for !4314 with mnowak/test-coverage-visualization-v9_16 in 6 seconds (queued for 8 seconds)
detached
Status Job ID Name Coverage
  Precheck
canceled #1252716
amd64 linux
autoreconf

00:00:05

canceled #1252718
amd64 linux
clang-format

00:00:05

canceled #1252719
amd64 linux
coccinelle

00:00:04

canceled #1252720
amd64 linux
danger

00:00:04

canceled #1252717
amd64 linux
misc

00:00:05

 
  Build
canceled #1252734
amd64 linux
clang:asan

canceled #1252738
amd64 linux
clang:buster:amd64

canceled #1252741
amd64 libvirt
clang:freebsd11:amd64

canceled #1252742
amd64 libvirt
clang:freebsd12:amd64

canceled #1252743
amd64 libvirt
clang:openbsd:amd64

canceled #1252736
amd64 linux
clang:tsan

canceled #1252721
amd64 linux
gcc:alpine3.12:amd64

canceled #1252733
amd64 linux
gcc:asan

canceled #1252726
amd64 linux
gcc:buster:amd64

canceled #1252722
amd64 linux
gcc:centos6:amd64

canceled #1252723
amd64 linux
gcc:centos7:amd64

canceled #1252724
amd64 linux
gcc:centos8:amd64

canceled #1252732
amd64 linux
gcc:focal:amd64

canceled #1252737
amd64 linux
gcc:mutexatomics

canceled #1252728
amd64 linux
gcc:out-of-tree

canceled #1252727
amd64 linux
gcc:sid:amd64

canceled #1252729
i386 linux
gcc:sid:i386

canceled #1252739
amd64 linux
gcc:softhsm2.4

canceled #1252740
amd64 linux
gcc:softhsm2.6

canceled #1252725
amd64 linux
gcc:stretch:amd64

canceled #1252735
amd64 linux
gcc:tsan

canceled #1252730
amd64 linux
gcc:tumbleweed:amd64

canceled #1252731
amd64 linux
gcc:xenial:amd64

canceled #1252745
amd64 windows
msvc:windows:amd64

00:00:03

canceled #1252744
amd64 linux
nolibtool:sid:amd64

 
  Unit
canceled #1252758
amd64 linux
unit:clang:asan

canceled #1252762
amd64 linux
unit:clang:buster:amd64

canceled #1252765
amd64 libvirt
unit:clang:freebsd11:amd64

canceled #1252766
amd64 libvirt
unit:clang:freebsd12:amd64

canceled #1252760
amd64 linux
unit:clang:tsan

canceled #1252746
amd64 linux
unit:gcc:alpine3.12:amd64

canceled #1252757
amd64 linux
unit:gcc:asan

canceled #1252751
amd64 linux
unit:gcc:buster:amd64

canceled #1252747
amd64 linux
unit:gcc:centos6:amd64

canceled #1252748
amd64 linux
unit:gcc:centos7:amd64

canceled #1252749
amd64 linux
unit:gcc:centos8:amd64

canceled #1252756
amd64 linux
unit:gcc:focal:amd64

canceled #1252761
amd64 linux
unit:gcc:mutexatomics

canceled #1252752
amd64 linux
unit:gcc:sid:amd64

canceled #1252753
i386 linux
unit:gcc:sid:i386

canceled #1252763
amd64 linux
unit:gcc:softhsm2.4

canceled #1252764
amd64 linux
unit:gcc:softhsm2.6

canceled #1252750
amd64 linux
unit:gcc:stretch:amd64

canceled #1252759
amd64 linux
unit:gcc:tsan

canceled #1252754
amd64 linux
unit:gcc:tumbleweed:amd64

canceled #1252755
amd64 linux
unit:gcc:xenial:amd64

canceled #1252767
amd64 linux
unit:nolibtool:sid:amd64

 
  System
canceled #1252780
amd64 linux
system:clang:asan

canceled #1252784
amd64 linux
system:clang:buster:amd64

canceled #1252787
amd64 libvirt
system:clang:freebsd11:amd64

canceled #1252788
amd64 libvirt
system:clang:freebsd12:amd64

canceled #1252782
amd64 linux allowed to fail
system:clang:tsan

canceled #1252768
amd64 linux
system:gcc:alpine3.12:amd64

canceled #1252779
amd64 linux
system:gcc:asan

canceled #1252773
amd64 linux
system:gcc:buster:amd64

canceled #1252769
amd64 linux
system:gcc:centos6:amd64

canceled #1252770
amd64 linux
system:gcc:centos7:amd64

canceled #1252771
amd64 linux
system:gcc:centos8:amd64

canceled #1252778
amd64 linux
system:gcc:focal:amd64

canceled #1252783
amd64 linux
system:gcc:mutexatomics

canceled #1252774
amd64 linux
system:gcc:sid:amd64

canceled #1252775
i386 linux
system:gcc:sid:i386

canceled #1252785
amd64 linux
system:gcc:softhsm2.4

canceled #1252786
amd64 linux
system:gcc:softhsm2.6

canceled #1252772
amd64 linux
system:gcc:stretch:amd64

canceled #1252781
amd64 linux allowed to fail
system:gcc:tsan

canceled #1252776
amd64 linux
system:gcc:tumbleweed:amd64

canceled #1252777
amd64 linux
system:gcc:xenial:amd64

canceled #1252789
amd64 linux
system:nolibtool:sid:amd64

 
  Docs
canceled #1252790
amd64 linux
docs

 
  Postcheck
canceled #1252794
amd64 linux
cppcheck

canceled #1252791
amd64 linux
flake8

canceled #1252795
amd64 linux
gcov

canceled #1252792
amd64 linux
pylint

canceled #1252793
amd64 linux
scan-build