[#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