Add unit tests for the fixtures

[tkilias]

Currently, we mainly use integration tests for the fixtures. These are expensive, and with the SaaS tests, also unstable. We should think about adding unit tests for the fixtures. I see two possible strategies:

1. Extract the logic of the fixtures into ordinary functions and test these functions with mocks.
2. Encapsulate objects or functions that do expensive things into their own fixtures and overwrite these fixtures with mocks in the test