[1.7.x] Fixed #22771 -- Fixed test.Client.logout when using custom auth backend.

Backport of 50b9313e0a from master

tests/test_client_regress/auth_backends.py

@@ -0,0 +1,20 @@
+from django.contrib.auth.backends import ModelBackend
+
+from .models import CustomUser
+
+
+class CustomUserBackend(ModelBackend):
+
+    def authenticate(self, username=None, password=None):
+        try:
+            user = CustomUser.custom_objects.get_by_natural_key(username)
+            if user.check_password(password):
+                return user
+        except CustomUser.DoesNotExist:
+            return None
+
+    def get_user(self, user_id):
+        try:
+            return CustomUser.custom_objects.get(pk=user_id)
+        except CustomUser.DoesNotExist:
+            return None