Adds tests for new UiDebugOverlay features into testbed_ui

[kfc35]

Objective

• Add some recently expanded UI Debug functionality to the testbed New UiDebugOverlay features #21931

Solution

To testbed/ui.rs, Adds a second row of entities demonstrating:

1. border, padding, content box outlines with a rounded border radius
2. vertical scrollbar outline
3. horizontal scrollbar outline
4. bidirectional scrollbar outline

The scrollbars were taken from scroll.rs and I tried to strip them down to the barest code that shows them as outlines (so scrolling doesn’t work with them)

Testing

cargo run --example testbed_ui --features="bevy_ui_debug” -- debugoutlines does the trick

Showcase

If this screenshot / these new test cases expose(s) any bugs that are not intended behavior (i.e. why is the bottom left padding box’s corner also rounded when only the bottom right border radius was specified? Or why do the bidirectional scrollbars overlap the text?), please make an issue / let me know so that I can make an issue

Screenshot

[ickshonpe]

Two tweaks I'd like:

• The scrollbar examples are all 100 square, varying the dimensions will catch more issues.
• The scrollbars are non-functional but you could still set non-zero ScrollPositions on some of the axes.

Otherwise, looks good.