Minor test fixes

* Use dxpy instead of shelling out to "dx new project", "dx wait", etc.
  Considering that each setUp and tearDown used to invoke dx twice, this
  should save upwards of 300ms *per test*.
* Convert some print statements to test names.
* assertSubprocessFailure no longer prints out stderr unless the regexp
  assertion failed; it's likely that check_output already printed stderr
  out anyway, so such lines were duplicated.