Create custom egui_kittest::Node (#7138)

This adds a custom Node struct with proper support for egui types
(`Key`, `Modifiers`, `egui::Event`, `Rect`) instead of needing to use
the kittest / accesskit types.

I also changed the `click` function to do a proper mouse move / mouse
down instead of the accesskit click. Also added `accesskit_click` to
trigger the accesskit event. This resulted in some changed snapshots,
since the elements are now hovered.

Also renamed `press_key` to `key_press` for consistency with
`key_down/key_up`.

Also removed the Deref to the AccessKit Node, to make it clearer when to
expect egui and when to expect accesskit types.

* Closes #5705 
* [x] I have followed the instructions in the PR template

crates/egui_demo_lib/src/rendering_test.rs

@@ -737,8 +737,9 @@ mod tests {
                 });
 
             {
-                // Expand color-test collapsing header
-                harness.get_by_label("Color test").click();
+                // Expand color-test collapsing header. We accesskit-click since collapsing header
+                // might not be on screen at this point.
+                harness.get_by_label("Color test").click_accesskit();
                 harness.run();
             }