action_sheet test: Create message with the appropriate flag.

Otherwise this message wouldn't be expected to appear in the
messages fetched for MentionsNarrow.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

test/widgets/action_sheet_test.dart

@@ -388,7 +388,7 @@ void main() {
     });
 
     testWidgets('not offered in MentionsNarrow (composing to reply is not yet supported)', (tester) async {
-      final message = eg.streamMessage();
+      final message = eg.streamMessage(flags: [MessageFlag.mentioned]);
       await setupToMessageActionSheet(tester, message: message, narrow: const MentionsNarrow());
       check(findQuoteAndReplyButton(tester)).isNull();
     });