I normally try not to make changes "while we're in here" for fear of
introducing errors! But I guess the test cases are less critical, so
I've taken your review comments as a license to submit a revised patch
that:

* incorporates your suggestion to use assert_python_ok from
  test.script_helper, instead of subprocess.call;
* replaces the other uses of subprocess.call with
  assert_python_failure and adds a check on stdout;
* cleans up the assertion-testing code using the context manager form
  of unittest.TestCase.assertRaises.

I've signed and submitted a contributor agreement as requested.