Add mock to test-requirements.txt

mock is used by the oslo incubator test fixtures in mockpatch. So this
patch ensures it gets installed by adding it to the test requirements
list.

Change-Id: Ib3a1a074143595247790084c04edfc36e19cd94f
diff --git a/test-requirements.txt b/test-requirements.txt
index 41a784e..5cdb819 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -5,3 +5,4 @@
 python-subunit
 oslo.sphinx
 mox>=0.5.3
+mock>=1.0