Fixed #8138 -- Changed django.test.TestCase to rollback tests (when the...

Fixed #8138 -- Changed django.test.TestCase to rollback tests (when the database supports it) instead of flushing and reloading the database.   This can substantially reduce the time it takes to run large test suites.

This change may be slightly backwards incompatible, if existing tests need to test transactional behavior, or if they rely on invalid assumptions or a specific test case ordering.  For the first case, django.test.TransactionTestCase should be used.  TransactionTestCase is also a quick fix to get around test case errors revealed by the new rollback approach, but a better long-term fix is to correct the test case.  See the testing doc for full details.

Many thanks to:
* Marc Remolt for the initial proposal and implementation.
* Luke Plant for initial testing and improving the implementation.
* Ramiro Morales for feedback and help with tracking down a mysterious PostgreSQL issue.
* Eric Holscher for feedback regarding the effect of the change on the Ellington testsuite.
* Russell Keith-Magee for guidance and feedback from beginning to end.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9756 bcc190cf-cafb-0310-a4f2-bffc1f526a37