Fixed #17948 -- Isolated auth tests from custom template loaders

Thanks andrey@kostenko.name for the report.

django/contrib/auth/tests/views.py

 import os
 import re
 
-from django.conf import settings
+from django.conf import global_settings, settings
 from django.contrib.sites.models import Site, RequestSite
 from django.contrib.auth.models import User
 from django.core import mail
@@ -23,7 +23,8 @@ from django.contrib.auth.forms import (AuthenticationForm, PasswordChangeForm,
         ('en', 'English'),
     ),
     LANGUAGE_CODE='en',
-    TEMPLATE_DIRS = (
+    TEMPLATE_LOADERS=global_settings.TEMPLATE_LOADERS,
+    TEMPLATE_DIRS=(
         os.path.join(os.path.dirname(__file__), 'templates'),
     ),
     USE_TZ=False,

django/test/signals.py

@@ -51,6 +51,13 @@ def clear_context_processors_cache(**kwargs):
         context._standard_context_processors = None
 
 
+@receiver(setting_changed)
+def clear_template_loaders_cache(**kwargs):
+    if kwargs['setting'] == 'TEMPLATE_LOADERS':
+        from django.template import loader
+        loader.template_source_loaders = None
+
+
 @receiver(setting_changed)
 def clear_serializers_cache(**kwargs):
     if kwargs['setting'] == 'SERIALIZATION_MODULES':