message test: Use hasAlertWord flag, where starred is inappropriate

UpdateMessageEvent has a `flags` field, but the API doc is pretty
clear that it's just meant to communicate changes in the mentioned
and alert-words flags:
  https://zulip.com/api/get-events#update_message
The separate event, UpdateMessageFlagsEvent, is what updates the
starred flag.

Author: chrisbobbe

test/model/message_test.dart

@@ -1440,7 +1440,7 @@ void main() {
       final originalMessage = eg.streamMessage(
         content: "<p>Hello, world</p>");
       final updateEvent = eg.updateMessageEditEvent(originalMessage,
-        flags: [MessageFlag.starred],
+        flags: [MessageFlag.hasAlertWord],
         renderedContent: "<p>Hello, edited</p>",
         editTimestamp: 99999,
         isMeMessage: true,