Thanks to mpasternak for the report and Tim Graham for the review.

Refs #23685.

Thanks Claude Paroz for assistance with debugging the tests.

This is the result of Christopher Medrela's 2013 Summer of Code project.

Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.

Also: Fixes #8579, fixes #3055, fixes #19844.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

* Removed trailing whitespace.
* Added newline to EOF if missing.
* Removed blank lines at EOF.
* Removed some stray tabs.

Thanks al_the_x for the report and fheinz for the draft patch.

It doesn't work as one might expect on a certain database backend where
autocommits_when_autocommit_is_off = True. That backend happens to be
popular for running tests.

This was a stealth option only used by the tests, and it isn't useful
any more since `atomic` provides nested transactions.

Fixed #20483.

The failure was caused by generating the same warning from two tests.
The second time the same warning was raised it was swallowed by the
"once" simplefilter of warnings.

If the fixture doesn't exist, loaddata will output a warning.

The fixture named "initial_data" is exceptional though; if it
doesn't exist, the warning is not emitted. This allows syncdb and
flush management commands to attempt to load it without causing
spurious warnings.

Thanks to Derega, ptone, dirigeant and d1ffuz0r for contributions
to the ticket.