Comment 8 for bug 1199536

The change was submitted to oslotest in https://review.openstack.org/#/c/74861/ and in that review I said:

I discussed this change with lifeless and he indicated that he would be very happy to have something like this (without committing to accepting this exact patch after only a quick review). Please submit a pull request to testtools (https://github.com/testing-cabal/testtools) and when it's committed we can bump the version of testtools OpenStack uses.