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: I80669a94cb9e3fa78e875b7b0bf28e461c5d56a0

openstack/networking-hyperv,stackforge/networking-hyperv,openstack/networking-hyperv