Move away from python setup.py test who is deprecated in pbr

https://docs.openstack.org/pbr/latest/user/features.html#test

Change-Id: Ie5f7cf0d8eefed2ee756114ef5a145fe151b11b2

tox.ini

@@ -34,7 +34,13 @@ commands =
   sphinx-build -W --keep-going -b html doc/source doc/build/html
 
 [testenv:cover]
-commands = python setup.py test --coverage --coverage-package-name=oslo_policy --testr-args='{posargs}'
+setenv =
+    PYTHON=coverage run --source oslo_policy --parallel-mode
+commands =
+  stestr run --slowest {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage report
 
 [flake8]
 show-source = True