Update SelectableRegion test for M3 (#129627)

## Description

This PR  fixes one selectable region test failure when switching to M3.
The failure is somewhat tricky because it is related to the M3 typography (line height set to 1.43).

## Related Issue

fixes https://github.com/flutter/flutter/issues/129626

## Tests

Updates 1 test.

packages/flutter/lib/src/widgets/selectable_region.dart

@@ -2052,15 +2052,15 @@ abstract class MultiSelectableSelectionContainerDelegate extends SelectionContai
       if (globalRect.contains(event.globalPosition)) {
         final SelectionGeometry existingGeometry = selectables[index].value;
         lastSelectionResult = dispatchSelectionEventToChild(selectables[index], event);
+        if (index == selectables.length - 1 && lastSelectionResult == SelectionResult.next) {
+          return SelectionResult.next;
+        }
         if (lastSelectionResult == SelectionResult.next) {
           continue;
         }
         if (index == 0 && lastSelectionResult == SelectionResult.previous) {
           return SelectionResult.previous;
         }
-        if (index == selectables.length - 1 && lastSelectionResult == SelectionResult.next) {
-          return SelectionResult.next;
-        }
         if (selectables[index].value != existingGeometry) {
           // Geometry has changed as a result of select word, need to clear the
           // selection of other selectables to keep selection in sync.

packages/flutter/test/widgets/selectable_region_test.dart

@@ -1570,7 +1570,6 @@ void main() {
       final UniqueKey outerText = UniqueKey();
       await tester.pumpWidget(
         MaterialApp(
-          theme: ThemeData(useMaterial3: false),
           home: SelectableRegion(
             focusNode: FocusNode(),
             selectionControls: materialTextSelectionControls,
@@ -1595,8 +1594,13 @@ void main() {
         ),
       );
       final RenderParagraph paragraph = tester.renderObject<RenderParagraph>(find.descendant(of: find.byKey(outerText), matching: find.byType(RichText)).first);
+
+      // Adjust `textOffsetToPosition` result because it returns the wrong vertical position (wrong line).
+      // TODO(bleroux): Remove when https://github.com/flutter/flutter/issues/133637 is fixed.
+      final Offset gestureOffset = textOffsetToPosition(paragraph, 125).translate(0, 10);
+
       // Right click to select word at position.
-      final TestGesture gesture = await tester.startGesture(textOffsetToPosition(paragraph, 125), kind: PointerDeviceKind.mouse, buttons: kSecondaryMouseButton);
+      final TestGesture gesture = await tester.startGesture(gestureOffset, kind: PointerDeviceKind.mouse, buttons: kSecondaryMouseButton);
       addTearDown(gesture.removePointer);
       await tester.pump();
       await gesture.up();