Fixed #17787 -- Documented reset caches by setting_changed signal

    the original ``LoginTestCase`` is still equally affected by the

    decorator.

.. note::

When overriding settings, make sure to handle the cases in which your app's

code uses a cache or similar feature that retains state even if the

setting is changed. Django provides the

:data:`django.test.signals.setting_changed` signal that lets you register

callbacks to clean up and otherwise reset state when settings are changed.

    Note that this signal isn't currently used by Django itself, so changing

    built-in settings may not yield the results you expect.

Django itself uses this signal to reset various data:

=========================== ========================

Overriden settings          Data reset

=========================== ========================

USE_TZ, TIME_ZONE           Databases timezone

TEMPLATE_CONTEXT_PROCESSORS Context processors cache

TEMPLATE_LOADERS            Template loaders cache

SERIALIZATION_MODULES       Serializers cache

LOCALE_PATHS, LANGUAGE_CODE Default translation and loaded translations

=========================== ========================

Emptying the test outbox

~~~~~~~~~~~~~~~~~~~~~~~~