tests: Clean up base test
tests: Clean up base test

This base test isn't actually used by anything but we're going to change
that shortly. Before we do, address a longstanding TODO and make it
subclass 'oslotest.BaseTestCase'. Because 'oslotest.BaseTestCase'
already configures all of the fixtures we're currently using, our own
base test class ends up being a simple wrapper. This will also change in
the future.

Change-Id: I110392586fb032507d122ae9a412a6bba7c5ff2d
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/oslo.db,openstack/oslo.db