CI: add test coverage results

This runs the coverage tools to produce HTML pages listing the code lines /
functions / branches hit/totalled by the test suite.

Nowadays Gitlab has some Cobertura support itself:
https://docs.gitlab.com/ee/user/project/merge_requests/test_coverage_visualization.html



lcov is needed for the HTML report, gcovr is needed for the Cobertura
report. 'ninja clean' must be removed, otherwise it deletes the coverage
files before they are analysed.

Seeing the test suite code coverage is really interesting. It can guide
designing tests. If Gitlab MRs show the coverage in diff view, it shows
if new code actually gets executed in CI.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>