[flutter_test] export fake from flutter_test (#68032)

Fake has been moved from mockito to test to facilitate NNBD, re-export this from flutter_test

packages/flutter_test/lib/src/test_compat.dart

@@ -21,6 +21,9 @@ import 'package:test_api/src/backend/state.dart'; // ignore: implementation_impo
 // ignore: deprecated_member_use
 import 'package:test_api/test_api.dart';
 
+// ignore: deprecated_member_use
+export 'package:test_api/fake.dart' show Fake;
+
 Declarer? _localDeclarer;
 Declarer get _declarer {
   final Declarer? declarer = Zone.current[#test.declarer] as Declarer?;