Added test_clicking_on_content_rating and removed hardcoded test app

This is for issue https://github.com/mozilla/marketplace-tests/issues/525

Also made some changes to the Details page, not to use hardcoded test app
This bunch of code (from here to line 38) is also repeated in this file a number of times. This seems like a candidate to go into `base_test` or if not at least refactored into a separate method in this file that can then be called from each test.