Add support for scrolling via accesskit / kittest (#7286)

I need to scroll in a snapshot test in my app, and kittest had no
utilities for this. Event::MouseWheel is error prone. This adds support
for some accesskit scroll actions, and uses this in kittest to add
helpers to scroll to a node / scroll the scroll area surrounding a node.

The accesskit code says down/up/left/right `Scrolls by approximately one
screen in a specific direction.`. Unfortunately it's difficult to get
the size of a "screen" (I guess that would be the size of the containing
scroll area)where I implemented the scrolling, so for now I've hardcoded
it to 100px. I think scrolling a fixed amount is still better than not
scrolling at all.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_kittest/tests/tests.rs

@@ -1,5 +1,5 @@
-use egui::{Modifiers, Vec2, include_image};
-use egui_kittest::Harness;
+use egui::{Modifiers, ScrollArea, Vec2, include_image};
+use egui_kittest::{Harness, SnapshotResults};
 use kittest::Queryable as _;
 
 #[test]
@@ -81,3 +81,60 @@ fn should_wait_for_images() {
 
     harness.snapshot("should_wait_for_images");
 }
+
+fn test_scroll_harness() -> Harness<'static, bool> {
+    Harness::builder()
+        .with_size(Vec2::new(100.0, 200.0))
+        .build_ui_state(
+            |ui, state| {
+                ScrollArea::vertical().show(ui, |ui| {
+                    for i in 0..20 {
+                        ui.label(format!("Item {i}"));
+                    }
+                    if ui.button("Hidden Button").clicked() {
+                        *state = true;
+                    };
+                });
+            },
+            false,
+        )
+}
+
+#[test]
+fn test_scroll_to_me() {
+    let mut harness = test_scroll_harness();
+    let mut results = SnapshotResults::new();
+
+    results.add(harness.try_snapshot("test_scroll_initial"));
+
+    harness.get_by_label("Hidden Button").scroll_to_me();
+
+    harness.run();
+    results.add(harness.try_snapshot("test_scroll_scrolled"));
+
+    harness.get_by_label("Hidden Button").click();
+    harness.run();
+
+    assert!(
+        harness.state(),
+        "The button was not clicked after scrolling."
+    );
+}
+
+#[test]
+fn test_scroll_down() {
+    let mut harness = test_scroll_harness();
+
+    let button = harness.get_by_label("Hidden Button");
+    button.scroll_down();
+    button.scroll_down();
+    harness.run();
+
+    harness.get_by_label("Hidden Button").click();
+    harness.run();
+
+    assert!(
+        harness.state(),
+        "The button was not clicked after scrolling down. (Probably not scrolled enough / at all)"
+    );
+}