bevyengine#12803 Changes (squashed)

Apply rustfmt
Move test_initialization next to other tests
Add doc comment to is_root_node_pair_valid
Move helper methods into the single test case where they are used
Add tests for helper methods
Remove trait helpers to reduce complexity of tests
Mark specific test functions as unreachable
Move ui_surface test only methods into mod tests as trait
Fix tests after rebase
Add tests for bevy_ui/layout/ui_surface
Widen type for parameter children in UiSurface::update_children
Add missing asserts and Debug fields in UiSurface from bevyengine#12268 and bevyengine#12698