Reduce test spam by using unittest output buffering

[awwad]

Thanks go to @lukpueh for this helpful tip.

This is a succinct replacement for PR #819.

After merge, stdout from the code being tested should only appear if the test has failed or errored out.

This functionality is provided by unittest.TextTestRunner argument buffer=True.
This functions like the --buffer command line argument listed here

[lukpueh]

Approving the patch... :)

Please fix travis py27 build failure before merging though. I don't think it is related to your changes, unless Python2's SimpleHttp, from which the failure seems to stem, does not like unittest's standard stream overriding? OTOH, the tests pass locally on my machine.

I'm also linking our discussion from in-toto/in-toto#240. It might be helpful if you ever come back here, e.g. because you want to enhance unittest output.

[awwad]

I looked over it, and I ran it again and it passed. Usually, the cause of something like the error there is an inadequate delay to let the http/s server or (in this case) proxy start up. In this case, though, the delay of 4s seems like plenty. I've also seen rare port collisions, but that's already corrected for here (unless multiple test modules are running in parallel for some reason, perhaps). The proxy may not have been ready, or some other issue may have occurred with the proxy server startup. I'm going to tolerate low rates of test failures if they occur due to a test proxy connection issue (and succeed when re-run).