[2.2.x] Refs CVE-2020-13254 -- Fixed cache.tests when KEY_PREFIX is d…

…efined.

Follow up to 2c82414.

Backport of 229c9c6 from master

tests/cache/tests.py

@@ -1256,7 +1256,7 @@ def _perform_invalid_key_test(self, key, expected_warning):
         Whilst other backends merely warn, memcached should raise for an
         invalid key.
         """
-        msg = expected_warning.replace(key, ':1:%s' % key)
+        msg = expected_warning.replace(key, cache.make_key(key))
         with self.assertRaisesMessage(InvalidCacheKey, msg):
             cache.set(key, 'value')