Add mock to test-requirements.txt
Add mock to test-requirements.txt

Mention mock explicitly rather than relying on it to be installed
as a transitive dependency, so that we have control on the mock
versions allowed.

Partial-Bug: #1476665

Change-Id: Id734c3d7b7ef1ab8d97a13edb1de006c029c22a0

openstack/oslo.db,magic0704/oslo.db,openstack/oslo.db