Add integration tests

On second thought, this is no more or less informative than the other XBlock readmes on the matter. The general 'testing an xblock' tutorial is still the best resource on this, and this answer is technically correct even if oversimplified. We'd be likely to do a lot of copy-paste if we did the full tutorial each time. This does beg the question on 'what's the best way to get this information across', but I don't think this PR has to answer that.