django - specify database for TestCase fixtures

I have two databases that my site uses and I have an app that uses both of them. I need to write a TestCase that loads fixtures for both databases. I use a DB router, which works fine in production, but in the testing framework, Django insists on using the 'default' database for all fixtures, even for models that specify the other database. How do I tell Django to run a fixture against another database?

My TestCase is defined list:

class VerifierTestCase(TestCase):
    fixtures = ['zipcodes_test.json', 'all_states.json', 'wtf.json']
    multi_db = True