Fix action heartbeat checker test

* With the recent update of the cachetools library the internal
  behavior of the cache became different and we made some assumptions
  in the test about how it should work. We did mocking for this.
  Now we have to modify it to reflect the latest behavior.

Change-Id: If76690481b5e0b88c034e35ef17657d59aaa4b4d

mistral/tests/unit/engine/test_action_heartbeat_checker.py

@@ -92,6 +92,11 @@ class ActionHeartbeatCheckerTest(base.EngineTestCase):
         '__getitem__',
         mock.MagicMock(side_effect=KeyError)
     )
+    @mock.patch.object(
+        cachetools.LRUCache,
+        '__contains__',
+        mock.MagicMock(return_value=False)
+    )
     def test_fail_action_with_missing_heartbeats_wf_spec_not_cached(self):
         wf_text = """---
         version: '2.0'