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: I9edf39f404f70f71e722e900f4ba8ee9676329e2

openstack/python-ironicclient,openstack/python-ironicclient