Fix broken file attachment and screenshot tests.
Review Request #8145 — Created May 5, 2016 and submitted
The tests for file attachments and screenshots (especially with regards to
sandboxing) were broken on release-3.0.x. The reason here is that we were often
creating objects directly in ways which didn't quite set up all the necessary
relations. I've switched over to using the utilities furnished by our TestCase,
which fixes most of the issues, and simplified some of the code to fix the
rest.
Ran unit tests.
Tool: Pyflakes Processed Files: reviewboard/reviews/tests.py reviewboard/reviews/ui/tests.py Tool: PEP8 Style Checker Processed Files: reviewboard/reviews/tests.py reviewboard/reviews/ui/tests.py