Add method to change the region for test_clicking_on_content_rating #529

This is for issue https://github.com/mozilla/marketplace-tests/issues/529
It might make sense to encapsulate these three lines into a method like `go_to_debug_page`, which could then return the `Debug` object. You could then call that method from this one and not have to instantiate the `Debug` object in here, which is a bit ugly.