The example_user() function is specifically designed for
AARON, hamlet, cordelia, and friends, and it allows a concise
way of using their built-in user profiles. Eventually, the
widespread use of example_user() should help us with refactorings
such as moving the tests users out of the "zulip.com" realm
and deprecating get_user_profile_by_email.
In this commit we add a logout wrapper so as to enable developers
to just do self.logout instead of doing a post request at API
endpoint for logout. This is achieved by adding a wrapper function
for the Django's client.logout contained in TestCase. We add this
by extending ZulipTestCase to have a logout function.