When djblets.util got split up, most of the tests remained in

djblets.util.tests, which was getting pretty big. These have now been

moved out into more appropriate locations that match the modules they're

testing.
The djblets.db tests got split up into multiple files within thee

djblets/db/tests/ directory, in order to stay organized. Many of the

current and planned tests require a handful of extra model classes,

which was already making that file a bit confusing. With some work I'm

doing now, that'd get worse. By splitting this up, we can stay

organized.

Unit tests pass.