Make tests installable

[wedaly]

Hi,

In a project I'm working on, we used to import the OAuth2 tests from DRF and run them on our custom OAuth2 subclass. In upgrading to DRF 3.1 and django-rest-framework-oauth, we noticed that the tests package is no longer pip installed, so we cannot import the test cases.

This is where we're trying to run the tests: https://github.com/edx/edx-platform/blob/release-2015-09-09/openedx/core/lib/api/tests/test_authentication.py

Looking through the commit history for DRF, it looks like the tests package was moved out of the installed package when adding support for pytest. Would it be possible to support pytest and still include the tests package in the pip-installable distribution for django-rest-framework-oauth?

-- Will