Don't rely on testtools.tests

We can't import testtools.tests because although we depend on
testtools, its test-requirements may not be available to us. So
just "inline" what we need from testtools.tests.

See I33564b06e9a2b128e892f0b598cfc71f71a7794a and
I0a34ae7b7000760a1be105c8c082b8aaa8e079e4 for more history and
this issue.

Change-Id: I4d67aafb3dcdefa105782883cd044b09ba5ee966
Closes-bug: #1634972
1 file changed