Add code coverage support

This issue is currently blocked by Unity as they did not implement this yet. If you'd like to see this implemented here, cast your votes at https://feedback.unity3d.com/suggestions/expose-unit-test-coverage-information-in-unity-test-tools

Edit: above link died, unity closed feedback website in favor of forums, but look at the following good news (first comment in this issue)

• Make sure Code Coverage plugin is installed
• Add coverage report generation to test scripts
• Add coverage report as an artifact
• Add OpenCover Regex to .gitlab-ci.yml test jobs