Post-processing of Twisted logs requires overriding TestCase.run

[Jonathan Lange]

Currently, if you want to do post-processing of attached Twisted logs (using AsynchronousDeferredRunTest, aka ADRT), then you have to override TestCase.run(), since the Twisted logs aren't attached until *after* the ADRT is finished with user code.

We don't want to encourage overriding TestCase.run, so instead we should:

* Publish a fixture for capturing Twisted logs
* Use this fixture in ADRT
* Provide a means of disabling the automatic capturing of logs in ADRT (probably a new optional parameter)

Then users who want to do post-processing could do:

  self.addCleanup(post_process_logs)
  self.useFixture(CaptureTwistedLogs())

[Jonathan Lange (jml)]

https://github.com/testing-cabal/testtools/pull/171
https://github.com/testing-cabal/testtools/pull/172