ci: enable test coverage reports

Enables the generation of test coverage reports for the CI pipeline.

The coverage status is displayed automatically in the MR widget, the pipeline jobs report, and in the MR diff viewer.

MR Widget	Job Report	Diff Viewer

Having the test coverage visible without any extra steps required makes it immediately obvious if the changes are covered by tests, increasing confidence in the code quality.

An example of this in action can be viewed at this test MR.

Relevant:

• https://docs.gitlab.com/ee/ci/testing/test_coverage_visualization.html
• https://community.kde.org/Infrastructure/Continuous_Integration_System#Options