Fix 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: Iad31fa3772dbb904cb2f71934a8a93d2ef18fc3d
Merge "Fix testtools version and bump subunit to >=0.0.18"