Delegate TestWindow.updateSemantics to the wrapped SingletonFlutterWindow (#114733) (#114827)

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

Co-authored-by: Jason Simmons <jason-simmons@users.noreply.github.com>
diff --git a/packages/flutter_test/lib/src/window.dart b/packages/flutter_test/lib/src/window.dart
index 9fdaf23..03a2f28 100644
--- a/packages/flutter_test/lib/src/window.dart
+++ b/packages/flutter_test/lib/src/window.dart
@@ -464,6 +464,11 @@
   }
 
   @override
+  void updateSemantics(ui.SemanticsUpdate update) {
+    _window.updateSemantics(update);
+  }
+
+  @override
   void setIsolateDebugName(String name) {
     platformDispatcher.setIsolateDebugName(name);
   }