Refactoring of the tests for UUID support

Hi Matthew,

Here is a cleaner version of the tests. I couldn't get the fixture part to work, as they are cached and instantiated at the very beginning, and figuring out how to properly reload them seemed to be a lot of work without increasing the quality of the tests by a lot. So this still adds a good amount of tests without duplication of the model.

I also had to decrease the number of workers from 8 to 4 to get the tests to run locally for some reason. Otherwise I kept getting "too many connections". I'm not sure if this is because of the changed tests or my local setup. We'll see what the CI says.