Create tests and run them on CI

Would make me more comfortable depending on this package in a production environment if there was a test suite.

I'm not quite sure how to test everything, but some tests that run the patchwork examples would probably go a good distance in catching different errors that arise. The cowpatch tests might be of some inspiration.

You can configure github actions to run the tests on every commit / PR as well as on a scheduled basis to detect for example if a dependency update breaks something. Happy to help set up the actions.