Add tests

![http://media.tumblr.com/tumblr_m7or5ecmr41rter4a.gif](http://media.tumblr.com/tumblr_m7or5ecmr41rter4a.gif)

This project doesn't have any tests, so we should remedy that.
Oh I know what it was - earlier today I switched from `unittest.TestCase` to `django.test.TestCase`, and the latter _does_ clear the outbox for you, whereas the `unittest` one doesn't.  I'd forgotten about this by the time I made the change.