Use mock.call_count to assert the call counter

In patch https://review.openstack.org/#/c/65424 , the unit test for
test_base_filter.py is not using mock.call_count to identify the
call counter.

This patch was using call_count to check that the filter was only
called once.

Change-Id: I792c894c0414d11ab7461b6e5cc9366ced787ebb
Merge "Use mock.call_count to assert the call counter"