Errors detected by Topy (https://github.com/intgr/topy), all changes
verified by hand.
Backport of 11d453bcadad3c9a5ae35882792d0f1e8e8e7818 from master
Also moved its documentation to the 'advanced' section. It doesn't
belong to the 'overview'. Same for TransactionTestCase.reset_sequences.
When available_apps is set, after a TransactionTestCase, the database
is now totally empty. post_syncdb is fired at the beginning of the next
TransactionTestCase.
Refs #20483.
Thanks to Preston Timmons for the bulk of the work on the patch, especially
updating Django's own test suite to comply with the requirements of the new
runner. Thanks also to Jannis Leidel and Mahdi Yusuf for earlier work on the
patch and the discovery runner.
Refs #11077, #17032, and #18670.