Run unit tests from executable Python modules

When developing on a Linux VM inside a Windows host, all Python modules have
the executable flag on. Nose has work around for this when run on Windows
(https://github.com/nose-devs/nose/blob/master/nose/config.py#L26). In this
case, the Python intrepeter run inside the VM causing the work-around to fail.
This review request adds check for executable file permission and forces Nose
to run tests.

This is similar to Djblets review request 5325
(https://reviews.reviewboard.org/r/5325/).

Testing Done:
Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/5383/