A few cleanups and simplifications to the mem bio tests

trial's unittest doesn't actually have a main function, so stop trying to use it (so stop having a __main__ - doh)
improve/add test docstrings in a few places
factor the buffer flushing logic into a general `_loopback` method and then use it in the test; add another app-level exchange
  to the main mem bio test
1 file changed