auth: Improve logic for invalid GitHub emails.

This deletes the old mock-covered test for this, which was mostly
useless.  We have a much less messy test, which we extend to provide
the same test coverage the old one did.

While the result was the same before, this makes it more obvious.

zproject/backends.py

@@ -249,6 +249,7 @@ class SocialAuthMixin(ZulipAuthMixin):
             logging.warning(
                 "{} got invalid email argument.".format(self.auth_backend_name)
             )
+            return None
 
         strategy = self.strategy  # type: ignore # This comes from Python Social Auth.
         request = strategy.request