msglist test: Use hasAlertWord flag in a test that just needs some flag

In this test, it's not important that specifically the starred flag
is used; the hasAlertWord flag works just as well. It would be a bit
annoying to refactor this around the starred flag, when we add a
data structure for starred messages, coming up.

Author: chrisbobbe

test/model/message_list_test.dart

@@ -2202,12 +2202,12 @@ void main() {
       await checkApplied(
         mkEvent: (message) => UpdateMessageFlagsAddEvent(
           id: 1,
-          flag: MessageFlag.starred,
+          flag: MessageFlag.hasAlertWord,
           messages: [message.id],
           all: false,
         ),
         doCheckMessageAfterFetch:
-          (messageSubject) => messageSubject.flags.contains(MessageFlag.starred),
+          (messageSubject) => messageSubject.flags.contains(MessageFlag.hasAlertWord),
       );
     });
   });