test_events: Separate tests for send and update message events.

This is a prep commit to separate the single test
'test_stream_send_message_events' into two separate tests named
'test_stream_send_message_events' & test_stream_update_message_events'
to verify the events related to send and update message, respectively.

As a part of introducing two new user settings
* 'automatically_follow_topics_policy'
* 'automatically_unmute_topics_policy'
in the next commit, we will extend 'test_stream_send_message_events'.

This logical separation helps in avoiding a single, super-long test.

zerver/tests/test_events.py

@@ -550,6 +550,9 @@ class NormalActionsTest(BaseAction):
         check_message("events[0]", events[0])
         assert events[0]["message"]["avatar_url"] is None
 
+    def test_stream_update_message_events(self) -> None:
+        self.send_stream_message(self.example_user("hamlet"), "Verona", "hello")
+
         # Verify stream message editing - content only
         message = Message.objects.order_by("-id")[0]
         content = "new content"