Show JUnit reports in merge requests

GitLab provides the functionality to show failed tests in merge requests (see this). This nicely shows if any tests fail on a merge request, which don't fail on the master, or it just shows the failing tests, without having to go into the test logs or any such thing.

Suggestions

Catch provides additional arguments to catch_discover_tests, among those are reporters. We could have a global option, which would be activated while running CI builds, which automatically generate JUnit like reports and upload them as artifacts to GitLab-CI.

This is also a first try on how it looks, then we can decide if we want it