INTPYTHON-435: investigate speeding up Django's TransactionTestCase/TestCase

[timgraham]

See the discussion in #179.

One possibility beyond that discussion is to have Django's test suite TestCase classes set available_apps. Currently, runtests.py only enforces available_apps for TransactionTestCase but for database backends like django-mongodb that don't support transactions, TestCase functions like TransactionTestCase with regards to flushing the database.

[Jibola]

https://jira.mongodb.org/browse/INTPYTHON-435