How to test a Flutter widget at a fixed point in time without a withClock closure

Inside testWidgets the ambient clock from package:clock is already fake, but it starts at whatever wall time the test began. Pin it for a whole suite by overriding runTest on a custom AutomatedTestWidgetsFlutterBinding installed from flutter_test_config.dart. Verified on Flutter 3.44.2, clock 1.1.2, fake_async 1.3.3.