Separating unittest per connectivity needs

Our unittests so far are offline, testing methods which do not require
Internet connectivity. Let's add some more tests which actually download
stuff, and put those in a different TestCase.
Add test data files

To test the methods which retrieve files from Wikimedia Commons,
we need some data files to test against.
They are made available to all unittests using setUp()
Add unittest for get_thumbnail_of_file()

This unittest tests the methods which retrieve the file thumnbail
contents form Wikimedia Commons against the contents of our test data files.
Raise Exception if download file thumbail fails

If the download fails for whatever reason, let's raise an Exception too
(and pass to it whatver error message we got) instead of silencing it.
Add unit test for get_thumbnail_of_file failure

In 5406345 we changed the behaviour of get_thumbnail_of_file to raise
an Exception if something goes wrong. Let's test that too.