Cleanup test-requirements
Cleanup test-requirements

python-subunit is not used directly anywhere
and it is dependency of both testrepository
and os-testr
(probably was used by some tox wrapper script before)

Change-Id: Ie8a7a8879e1a29d935538a288f03e0a281883160

openstack/solum,openstack/solum,stackforge/solum,stackforge/solum