Improve support for discovery on testtools.tests.

Our tests currently import test_with_with into test_testcase so that
we could still run tests on python2.5. Since we no longer support
2.5, we don't need this hack. Its a hack because discovery will
import test_with_with directly itself, and thus we triggered
a duplicate test id error when attempting to use discover on the
testtools test base.

Change-Id: I027929ab4b0df350dd18ab01ffd5c493fe35e101
Make discover run the same tests as 'make test'.

We use test parameterisation in test_deferredruntest, but hadn't
defined a load_tests hook to inform discover of that.

Change-Id: Ib32d7cf88b1a9e311476a5286357e46e65de1f1f