Move away from most fixtures on all non-webapi apps.

This removes the usage of the test_reviewrequests and most of the
test_scmtools fixtures from the non-webapi unit tests. Instead, these
test suites now use the new TestCase class's utility methods for
creating test objects.

With the test suite as it originally was, on my machine, it took roughly
159 seconds to complete. With this change, it's down to ~143. After my
upcoming change, which also removes data from the test_scmtools fixture,
the time drops to ~88.
Move away from most fixtures for the webapi tests.

This removes the usage of the test_reviewrequests and most of the
test_scmtools fixtures from the webapi unit tests. Instead, these test
suites now use the new TestCase class's utility methods for creating
test objects.

It also removes a lot of the older utility methods for creating dummy
objects that went through the API. Each of these requests contributed
50-100ms of time, which added up.

With the test suite as it originally was, on my machine, it took roughly
159 seconds to complete. After this and the previous changes, it drops
to around ~88.

Unit tests are much clearer now. Instead of having to know how fixtures
are set up, or tracing through a bunch of API requests, you can just
read the unit tests and see what they're doing.

In total, 1,834 lines of code have been deleted in these changes.