Improve testing

[edan-bainglass]

Our tests module could likely be made more efficient, especially once (and if 🤞) #802 is merged.

A couple of ideas:

1. Separate logic (models) and UI (widgets) testing
2. Combine related tests with shared content in class-based pytest modules to reduce test setup overhead

[danielhollas]

@edan-bainglass FYI in terms of speed have a look at #766, there's likely a lot of low-hanging fruit.

[edan-bainglass]

Thanks @danielhollas. Will start addressing this soon!