properly use settings_tests for tests

This is a very small step towards django1.7 (and its app-loading).
I'm not sure we need this to be updated: in the fixtures, we use mostly very old versions, like 4.0 or so. All the tests have been passing for quite a while now, even though the product details are updated every six weeks.

If we do need a specific (newer) version for a test, we'll have to update this here at the same time.