Add `egui_kittest` tests

[dmackdev]

Once egui_kittest is published, it would be great to use it for:

• Implementing tests that don't rely on the on_render render hook,
• Testing click interactions with the JsonTree,
• Snapshot tests for visual regressions.

As egui_kittest relies on accessibility labels, these would need to be added to the toggle buttons, and in a way that specifies their JSON pointer string. This should be behind a feature flag for this crate.