GitHub Action to Test Packages

To make sure we don't break something, there should be some recipe to test each and every package.

Perhaps a `test` script in the package folder that prints `ok` or anything else depending.

Idea is that we'd make sure every package has their `test` file run cleanly before merging.

Start by inspecting the package folders and any existing GitHub Actions configuration. Determine how each package exposes its test file and what a successful run should report. Done means the workflow runs every package test before merging and clearly fails when one does not pass.

Vom Indexierungsmodell aus dem Issue-Text verfasst.