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