Add unit tests

[PoignardAzur]

Masonry has pretty good unit testing tooling, but a lot of unit tests are still missing.

Ideally, we're aiming for 100% coverage (with opt-outs) for at least every widget and the WidgetPod code. Each widget should have tests covering:

• Creating the widget
• Displaying the widget
• Mutating the widget with WidgetMut.
• If the widget is interactive (eg a button, a textbox), interacting with it.

The first step would be to go over the code (potentially using cargo tarpaulin) and list all the places missing unit tests.