Use plain asserts and pytest-mock in tests/unit/test_backend.py (#116)

Author: cognifloyd

test-requirements.txt

@@ -4,6 +4,7 @@ mock==5.1.0
 pylint~=3.1.0
 pylint-plugin-utils>=0.4
 pytest
+pytest-mock
 st2-auth-backend-flat-file
 st2flake8>0.1
 tox