WIP: Generic Tests

[TimoImhof]

This PR aims to refactor the test suite into a more generic version.

The following points are to be covered (subject to change; will become more granular):

• [ ] Relocate edgecases due to model peculiarities out of the tests and into the respective model test classes to keeps the tests generic
• [ ] Add more documentation

This should make it easier to add new models to the existing test suite in the future.