Delay for duration of time.
In the automated test environment (AutomatedTestWidgetsFlutterBinding,
typically used in flutter test), this advances the fake clock for the
period and also increases timeout (see addTime).
In the live test environment (LiveTestWidgetsFlutterBinding, typically
used for flutter run and for e2e), it is
equivalent as Future.delayed.
Future<void> delayed(Duration duration);