Drop tox from test-requirements.txt
Drop tox from test-requirements.txt

test-reqs.xt gets installed by tox, so the chicken is before the egg.
This blocks devstack when run with strict requirements, as tox isn't
listed in global-requirements.txt.

Change-Id: I780fffa1e8a830b598ac2cee963c79ccad154e6a

openstack/akanda-rug,stackforge/akanda-rug,openstack/akanda-rug,stackforge/akanda-rug