Sacado de aquí: https://stackoverflow.com/a/503435/3377046.
You can put your tests in a tests/ subdirectory of the app (rather than a tests.py file), and include a tests/models.py with the test-only models.
Then provide a test-running script (example) that includes your tests/ "app" in INSTALLED_APPS. (This doesn't work when running app tests from a real project, which won't have the tests app in INSTALLED_APPS, but I rarely find it useful to run reusable app tests from a project, and Django 1.6+ doesn't by default.)
(NOTE: The alternative dynamic method described below only works in Django 1.1+ if your test case subclasses TransactionTestCase - which slows down your tests significantly - and no longer works at all in Django 1.7+. It's left here only for historical interest; don't use it.)
At the beginning of your tests (i.e. in a setUp method, or at the beginning of a set of doctests), you can dynamically add "myapp.tests" to the INSTALLED_APPS setting, and then do this:
from django.core.management import call_command
from django.db.models import loading
loading.cache.loaded = False
call_command('syncdb', verbosity=0)
Then at the end of your tests, you should clean up by restoring the old version of INSTALLED_APPS and clearing the app cache again.
This class encapsulates the pattern so it doesn't clutter up your test code quite as much.