interpreter under test is being run in an environment that requires the use of
environment variables such as PYTHONHOME in order to function at all.
Adds a private test.script_helper._interpreter_requires_environment() function
to be used with @unittest.skipIf on stdlib test methods requiring this.
Python subprocess failure assertion error messages for easier debugging.
Adds a unittest for test.script_helper to confirm that this code works as
it is otherwise uncovered by an already passing test suite that uses it. :)
