Extract database teardown from DiscoverRunner, to better support third-party test runners

[Carl Meyer]

Currently if a third-party test runner wants access to the default database teardown code, it has to instantiate a DiscoverRunner in order to call its teardown_databases method. This logic should be extracted into a standalone utility, like setup_databases already is.

Fortunately, Andreas Pelme has already done this in ​https://github.com/django/django/pull/6872 -- just filing a supporting Trac ticket to properly observe the forms.

[Tim Graham]

Left comments for improvement on the PR.

[Tim Graham <timograham@…>]

In e76981b4:

Fixed #26840 -- Added test.utils.setup/teardown_databases().

[Tim Graham <timograham@…>]

In 9463a7a:

Refs #26840 -- Removed django.test.runner.setup_databases() per deprecation timeline.

[Tim Graham <timograham@…>]

In f6d8b0c4:

Refs #26840 -- Corrected SQLite connection mocking in a setup_databases() test.

The test was expecting connections used by DiscoverRunner.setup_databases()
to be the ones defined in django.test.runner but this doesn't hold true
since this method was made a proxy of django.test.utils.setup_databases.

This broke the TransactionTestCase.serialized_rollback feature in the test
suite because calls to create_db_test() cleared the test data persisted on
connections objects.

Added an assertions to prevent this from happening again.