Rework unit tests

[PoignardAzur]

Masonry currently has a bunch of tests that check basic features of the widget-handling code (eg that focus is properly updated when a textbox is disabled).

However, these tests depend on the Command event, which is being removed as part of the Winit refactor to simplify the code. Since making these tests work without commands is complex, and the features being tested are subject to change anyway, some of those tests will be temporarily commented-out.

On the long term, these tests should be added back. This involves multiple steps:

• Implement a Harness::get_widget_mut(WidgetId) method as a substitute for targeting changes through commands.
• Update various helper widgets to work with WidgetMut.
• Go over basic widget-handling features and documenting what should happen in what conditions. For each expected behavior, create or repurpose a test.

I expect the steps to be implemented some time shortly after the Winit refactor.