Unpin testtools version and bump subunit to >=0.0.18

The problem of incompatibility between subunit<0.0.18 and
testtools>=0.9.35 was fixed in subunit>=0.0.18:
https://launchpad.net/subunit/trunk/0.0.18

Additionally, subunit is now depends on testtools>=0.9.34

Change-Id: If74785f9d9a59a9d6dc534b02f25ad041ee777cb