ci: Add lcov configuration to ignore g_return_*if_fail()/g_assert*()

While we can’t add markers to the macro implementations to cause lcov to
ignore them automatically, we can change our lcov configuration to
ignore all calls to them.

See https://github.com/linux-test-project/lcov/issues/44.

This causes all the un-takeable branches and un-reachable assertions to
be ignored by our code coverage, which bumps our statistics:
 • Lines: 74.9% → 74.8%
 • Functions: 82.3% → 82.3%
 • Branches: 53.3% → 64.2%

The rationale is that nobody should be testing programmer error
handling, as g_return_*if_fail() are used to guard against — so it’s not
reasonable to count missed branches like that in code coverage
statistics.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
9 jobs for <span class="ref-name">lcov-branch-coverage-tests</span> in 28 minutes and 34 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #117550
cross-android_api21_arm64

00:01:28

passed #117551
cross-android_api28_arm64

00:01:28

passed #117552
cross-mingw64

00:02:10

passed #117549
fedora-autotools-x86_64

00:05:40

passed #117548
fedora-x86_64

00:05:18

manual #117555
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #117553
win32
msys2-mingw32

00:10:28

passed #117554
win32
vs2017-x64

00:06:50

 
  Coverage
passed #117556
coverage

00:01:59

74.8%