Refactored file_storage tests to avoid depending on servers tests.
Documented TransactionTestCase.available_apps.

This is a private API but it's documented for Django contributors and
committers.
Defined available_apps in relevant tests.
Added TransactionTestCase.available_apps.

This makes Django's test suite significantly faster by reducing the
number of models for which content types and permissions must be created
and tables must be flushed in each non-transactional test.

Most of the credit goes to Anssi. He got the idea and did the research.