Shuffle around the location of various Djblets tests.

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.

Reviewed at https://reviews.reviewboard.org/r/6243/