This continues 6b4990f6 (gpython: Fix pymain to properly setup
sys.path[0]) - now we add test and verify that gpython actually sets
sys.path in the same way as underlying python does:

- don't add directory of gpython to sys.path
- for `gpython file` add to sys.path[0] realpath(dir(file)) instead of
  dir(file)  (see PySys_SetArgvEx for the reference, and else - if we do
  not make this change - added test breaks)
- for `gpython -m mod` add to sys.path[0] realpath('') instead of ''.
  This exactly matches PY3 behaviour, while PY2 behaviour is to add ''
  However PY3 behaviour

    * is more sane, and
    * fixes test_defer_excchain_traceback on py27-gevent

  -> so we stick to it.

Now all tests pass again.