modernize tests, use "mock"

[warner]

I'd like to clean up our unit tests.. there are a lot of archaic idioms that have crept in (partly because I'm not up-to-date on modern practices, partly because the tests were initially written before those practices were established). Here are some things that come to mind:

• use ​mock (which is kinda official now, added to stdlib as of py3.3) to shrink our 32 "Fake*" classes.
• replace our home-grown self.shouldFail with t.trial.unittest.assertFailure
• switch to inlineCallbacks to flatten the control-flow nest of helper functions