Make test_host_filters a NoDBTestCase

This is the final patch in the refactoring series. This changes the
test_host_filters.py file to contain just the test of the filter handler
system and cleans up the previously database-creating TestHostFilters
class.

The entire set of host filter unit tests (around 350 of them) now run in
less than 2 seconds, down from over a 45 seconds on my workstation.

Change-Id: I2f2518a08ac3f8c6addcd4055ee4c706c4c92e8e
Merge "Make test_host_filters a NoDBTestCase"