Include discover in test requirements.

* test-requirements.txt: Discover is needed for python26 tests.

Change-Id: If467058a326b285f394b63acf3432a98213f24fa
diff --git a/test-requirements.txt b/test-requirements.txt
index 65b91f0..b3e6e2c 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -3,6 +3,7 @@
 coverage
 sphinx
 docutils==0.9.1
+discover
 fixtures>=0.3.12
 python-subunit
 testrepository>=0.0.13