Add missing import of 'subunit' in test-requirements.txt

 tools/colorized.py import subunit
 however test-requirements.txt doesn't note this.

 Closes-Bug: #1224155

Change-Id: I4e1a8ee449c4fcf733116809de41e86d10f17e85
diff --git a/test-requirements.txt b/test-requirements.txt
index 6c313ca..1ede25e 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -2,3 +2,4 @@
 # needed for doc build
 docutils==0.9.1
 sphinx>=1.1.2
+python-subunit