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