Delegate TestWindow.updateSemantics to the wrapped SingletonFlutterWindow (#114733)

This restores the updateSemantics capability in TestWindow that had been
removed in https://github.com/flutter/flutter/pull/113382

packages/flutter_test/lib/src/window.dart

@@ -463,6 +463,11 @@ class TestWindow implements ui.SingletonFlutterWindow {
     platformDispatcher.onAccessibilityFeaturesChanged = callback;
   }
 
+  @override
+  void updateSemantics(ui.SemanticsUpdate update) {
+    _window.updateSemantics(update);
+  }
+
   @override
   void setIsolateDebugName(String name) {
     platformDispatcher.setIsolateDebugName(name);