Fixed #25251 -- Added cloning option to TransactionTestCase

[shangxiao]

https://code.djangoproject.com/ticket/25251
Forum post: https://forum.djangoproject.com/t/ticket-25251-opinions-required-cloning-the-test-db-as-an-alternative-to-rollback-emulation-in-transactiontestcase/29417

While looking at ticket 25251, I wondered whether cloning the test database per test in a TransactionTestCase was an option and found for my use cases it was a viable option. Experimentation & findings here: https://github.com/shangxiao/stupid-django-tricks/tree/master/clone_db_testcase

tl;dr:

• ✓ Cloning resolves the issue of missing data loaded via migrations when --keepdb is specified
• ✓ Cloning is much faster than rollback emulation aka serialized_rollback=True (tested on mysql, sqlite, postgres)
• TransactionTestCase.reset_sequences doesn't really apply with cloning as sequences are naturally reset when switching back to the test database
• TransactionTestCase.available_apps may no longer be necessary as cloning is already fast
• TransactionTestCase.fixtures are loaded per test but could potentially be loaded per class if we do a double clone 🤷‍♂️