TDD: Document Unit Tests and automated UI Testing

### Problem

Currently we are not using automated testing a lot - I _think_ this is mainly due to lack of knowledge. There's probably quite some potential in avoiding regressions, especially when we use a combination of automated UI and Unit testing.

### Proposal

Document the preferred way of doing UI testing as well as Unit Testing

### Prior Art (Optional)

- Gtk provides infrastructure for automated UI testing, not sure how this works though: https://valadoc.org/gtk4/Gtk.test_init.html
- There is Vest, a Unit Testing framework built with Vala: https://github.com/jorchube/vest
- There is Valadate as well which looks quite feature complete, even though it seems no longer to be maintained: https://github.com/chebizarro/valadate
- There is some documentation about testing in the GNOME Wiki (probably the foundation of Vest): https://wiki.gnome.org/Projects/Vala/TestSample
- And there is Gherkin - not sure if that's usefull in our case though: https://gist.github.com/chebizarro/6c949fafe1e9a2c59b26