reviewboard.search.tests was getting a bit hard to manage, and new

tests for new classes will soon be written. To prepare for this, this

change splits the test suite into 3 new files.
The test code is largely unchanged, with a few exceptions:

Missing/broken docstrings were added/corrected.
Some Django 1.6-specific code has been removed.
A KGB call has been updated for modern best practices.

Unit tests pass on Python 2.7 and 3.6 through 3.9.
Ensured I didn't have a stale tests.pyc or __pycache__ around.