test: Adapt to change in [MenuItemButton.onPressed] logic that broke …

…tests

Along with 2f0f469, this is a reminder that it'd be good to get
our repo added to Flutter's "customer tests" suite: zulip#239.

It's also another reminder among several that it'd be good to pin a
precise version of Flutter: zulip#15.

Discussion:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20Failing.20tests/near/1615254

Related: zulip#15
Related: zulip#239

test/widgets/action_sheet_test.dart

@@ -100,6 +100,7 @@ void main() {
       final quoteAndReplyButton = findQuoteAndReplyButton(tester);
       check(quoteAndReplyButton).isNotNull();
       await tester.tap(find.byWidget(quoteAndReplyButton!));
+      await tester.pump(); // [MenuItemButton.onPressed] called in a post-frame callback: flutter/flutter@e4a39fa2e
     }
 
     void checkLoadingState(PerAccountStore store, ComposeContentController contentController, {