tests: Limit email-based logins.

We now have this API...

If you really just need to log in
and not do anything with the actual
user:

    self.login('hamlet')

If you're gonna use the user in the
rest of the test:

    hamlet = self.example_user('hamlet')
    self.login_user(hamlet)

If you are specifically testing
email/password logins (used only in 4 places):

    self.login_by_email(email, password)

And for failures uses this (used twice):

    self.assert_login_failure(email)

zerver/tests/test_logging_handlers.py

@@ -74,8 +74,7 @@ class AdminNotifyHandlerTest(ZulipTestCase):
         handler.emit(record)
 
     def simulate_error(self) -> logging.LogRecord:
-        email = self.example_email('hamlet')
-        self.login(email)
+        self.login('hamlet')
         with patch("zerver.decorator.rate_limit") as rate_limit_patch:
             rate_limit_patch.side_effect = capture_and_throw
             result = self.client_get("/json/users")