[#299] Clear test data after each test is run

This might not be the best way to do it. It's too repetitive, but it's
better than leaving trash behind.

We might investigate running the tests inside a transaction, and rolling
back when we're done (is it done already?).

Conflicts:
	ckanext/jsonpreview/tests/test_preview.py
	ckanext/pdfpreview/tests/test_preview.py
	ckanext/reclinepreview/tests/test_preview.py
[#290] Remove datastore_active from results we got from resource_show in a test because datastore_active is not added to the original model