How can I use django's default test framework to test a module included in sys.path that is not a subdirectory of the django project?

Question

I have a django (1.8) site that is structured like this:

.../django_project/
    ./templates/
    ./manage.py
    ./<dir with settings,urls,etc>
.../django_app_project/
    ./app_name/
        ./<app files>
        ./test_app.py

To run the site, I set the environment variable PYTHONPATH=.../django_app_project/. This works perfectly when running the server, but causes problems when running the tests. Django's DiscoverRunner (django.test.runner.DiscoverRunner) does not notice .../Django_app_project/app_name/test_app.py. How can this be made to work without moving the app into the Django project? (It is outside for version control reasons)