Deprecate loading tests from models.py

[Alex Gaynor]

This is inconsistent with pretty much every other test tool, and really doesn't make a whole lot of sense.

[Russell Keith-Magee]

There is a reason that models.py was searched for tests -- it's to catch any doctests that are defined on model methods.

I'm not necessarily arguing against the decision to deprecate models.py as a source of tests, but given that it wasn't a completely arbitrary decision in the first place, it's worth documenting the specific consequences.

[Tim Graham]

I don't think this should be a documentation ticket as its scope is much larger than just that, correct?

[Carl Meyer <carl@…>]

In 9012833af857e081b515ce760685b157638efcef:

Fixed #17365, #17366, #18727 -- Switched to discovery test runner.

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.

[Florian Apolloner <florian@…>]

In e23a5f9a4730ddecb8f3950ee2936716f458c506:

Fixed a regression in the test runner loading of runtests.py.

Refs #17365, #17366, #18727.

[Florian Apolloner <florian@…>]

In 2bf403ecbd958bfb269794b36e61b69f0aede4cf:

Fixed a regression from e23a5f9a4730ddecb8f3950ee2936716f458c506.

Excluded postgis specific gis tests from other spatial databases.

Refs #17365, #17366, #18727.

[Tim Graham <timograham@…>]

In bf5430a20b65b3e76a2f8cd2580101e0baa59f82:

Removed django.test.simple and django.test._doctest per deprecation timeline.

refs #17365, #17366, #18727.