[1.5.x] Fixed #20075 -- Fixed session test fail when using a cache other than LocMemCache.

As override_settings was used after the initialization of the session backend,
we need to use a new session backend here.

Backport of 8c1cc4b3b0 from master

django/contrib/sessions/tests.py

@@ -468,6 +468,9 @@ class CacheSessionTests(SessionTestsMixin, unittest.TestCase):
         },
     }, SESSION_CACHE_ALIAS='sessions')
     def test_non_default_cache(self):
+        # Re-initalize the session backend to make use of overridden settings.
+        self.session = self.backend()
+
         self.session.save()
         self.assertEqual(get_cache('default').get(self.session.cache_key), None)
         self.assertNotEqual(get_cache('sessions').get(self.session.cache_key), None)