Add unit testing to github actions

[timvink]

This should get you started with unit testing. You can even schedule these to run every week (see https://github.com/timvink/mkdocs-git-revision-date-localized-plugin/blob/master/.github/workflows/scheduled_unittests.yml) so you are aware of anything breaking due to downstream updates

[fralau]

What is the difference between conftest.pyand test_builds.py ?

[timvink]

The conftest.py file serves as a means of providing fixtures for an entire directory.

https://docs.pytest.org/en/stable/reference/fixtures.html#conftest-py-sharing-fixtures-across-multiple-files

But didn't use it in this PR