migrate needs subunit >= 0.0.18
migrate needs subunit >= 0.0.18

The unit tests of subunit are failing with an older version of
subunit, so we need to fix the test-requirements.txt to reflect
the reality.

Change-Id: I786c70a02196b00fb0d96feb5fd1a0b5281f6672

openstack/sqlalchemy-migrate,andras-tim/sqlalchemy-migrate,openstack/sqlalchemy-migrate,stackforge/sqlalchemy-migrate,rcherrueau/sqlalchemy-migrate,rcherrueau/sqlalchemy-migrate,dannon/sqlalchemy-migrate