Simplify tests

[AlexWaygood]

Two of the explicit goals I had when starting this project was to learn more about pytest and to have a project with 100% test coverage. I succeeded in both those goals, but now the testing setup is so complex and fragile that it takes far longer to figure out how to write tests than it does to figure out how to write new features.

Ideally this repo would have simpler tests where I can remember how they when coming back to the repo after 6 months. Don't know if that's possible while still maintaining 100% test coverage.