test: Remove setUp method of tests.

If there is a better way of introducing other tests (in particular, the
preview tests), it would be nice to know.
test: Add unit test for preview pages.

There's an unfortunate fact regarding the way that we treat preview pages,
which has another undesired side-effect.

* The unfortunate fact is that we call BeautifulSoup for each preview
  video/link on the preview page.
* The side effect is that we end up having to need network access to in the
  unittest for the previews.

Actually, a consequence of the last point is that for a class that has a lot
of lectures (say, a long course), the test takes a lot of time.