Pass an async callback to testWidgets. (#33260)

The dart analyzer fails to warn about missing returns in function expressions due to a bug that has been
fixed but not yet rolled into Flutter (see
https://dart-review.googlesource.com/c/sdk/+/100301).  So it failed to catch that https://github.com/flutter/flutter/pull/33217 accidentally introduced a call to `testWidgets` that passed it a synchronous callback.

packages/flutter/test/painting/image_stream_test.dart

@@ -607,7 +607,7 @@ void main() {
     await tester.pump(const Duration(milliseconds: 200)); // emit 2nd frame.
   });
 
-  testWidgets('ImageStreamListener hashCode and equals', (WidgetTester tester) {
+  testWidgets('ImageStreamListener hashCode and equals', (WidgetTester tester) async {
     void handleImage(ImageInfo image, bool synchronousCall) { }
     void handleImageDifferently(ImageInfo image, bool synchronousCall) { }
     void handleError(dynamic error, StackTrace stackTrace) { }