test users: Add an escape char to a test username.

Changed the name of the test-user cordelia from `Cordelia Lear` to
`Cordelia, Lear's daughter`.

This change will enable us to test users with escape characters in
their names.

I also updated the Node, Puppeteer, Backend tests and Fixtures to
support this change.

zerver/tests/test_message_send.py

@@ -1642,7 +1642,7 @@ class StreamMessagesTest(ZulipTestCase):
         ).delete()
 
         def mention_cordelia() -> Set[int]:
-            content = "test @**Cordelia Lear** rules"
+            content = "test @**Cordelia, Lear's daughter** rules"
 
             user_ids = self._send_stream_message(
                 user=hamlet,