test: add missing test dependency

[nwatkins@smash ceph-ansible]$ virtualenv env
[nwatkins@smash ceph-ansible]$ env/bin/pip install -r tests/requirements.txt
[nwatkins@smash ceph-ansible]$ env/bin/python -c "import mock"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'mock'

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>

tests/requirements.txt

@@ -6,3 +6,4 @@ pytest==3.6.1
 notario>=0.0.13
 ansible~=2.7,<2.8
 netaddr
+mock