users: Add new user 'shiva' as realm moderator.

Note that at this point, it's not possible to create moderator users;
this just will make it easier to write tests for logic involving them
as we develop the feature.

zerver/lib/test_classes.py

@@ -379,6 +379,7 @@ Output:
         ZOE="ZOE@zulip.com",
         polonius="polonius@zulip.com",
         desdemona="desdemona@zulip.com",
+        shiva="shiva@zulip.com",
         webhook_bot="webhook-bot@zulip.com",
         welcome_bot="welcome-bot@zulip.com",
         outgoing_webhook_bot="outgoing-webhook@zulip.com",