Use --cover option when running CI tests

Together with a small regular expression this should show the test coverage in Gitlab.

For an example have a look at Pipeline #10076 on my fork.

Necessary Setup:

• Go to Settings -> CI/CD -> General pipelines -> Test coverage parsing.
• Add \d+.\d+\%\s+\|\s+Total and save your changes.

From now on there should be coverage numbers next to the unit-testing jobs and also at merge requests.

(Hint: See a little bit down from here.)