Add a new base TestCase class with helpful factory methods.
This introduces a new reviewboard.testing app with a new base TestCase

class that's intended to be used in other tests. This class provides a

handful of methods for creating various models we use in tests, along

with default data (which can be overridden by the callers).
This will allow us to reduce our dependence on most fixtures, which will

speed up tests, and make tests more readable.

Used this with a couple upcoming changes that eliminates the

test_reviewrequests and much of the test_scmtools fixtures.