This should detect when we're downloading a small text file by accident.
From now on, test_download will run these tests. That means we benefit not only from the networking setup in there, but also from the other tests (for example test_all_urls to find problems with _VALID_URLs).
This failed multiple times, and the md5sum is not that important anyways.
Previously, these extractors all defined their own suitable methods. No test_all_urls tests that everything is in order, automatically :)