Run tests via GitHub actions

[simonharrer]

• run tests on each commit and notify if tests fail
• run tests before each release to ensure that only releases with successful tests can be built

[Morl99]

Notifications should be setup using the default GitHub notification mechanisms: https://help.github.com/en/actions/building-and-testing-code-with-continuous-integration/about-continuous-integration#notifications-for-workflow-runs