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_hotspots.py

@@ -47,7 +47,7 @@ class TestHotspots(ZulipTestCase):
 
     def test_hotspots_url_endpoint(self) -> None:
         user = self.example_user('hamlet')
-        self.login(user.email)
+        self.login_user(user)
         result = self.client_post('/json/users/me/hotspots',
                                   {'hotspot': ujson.dumps('intro_reply')})
         self.assert_json_success(result)