[test] save packets as pcap file in thread-cert tests (#2965)

tests/scripts/thread-cert/test_service.py

@@ -85,8 +85,8 @@ class Test_Service(unittest.TestCase):
     def tearDown(self):
         for node in list(self.nodes.values()):
             node.stop()
-        del self.nodes
-        del self.simulator
+            node.destroy()
+        self.simulator.stop()
 
     def hasAloc(self, node_id, service_id):
         for addr in self.nodes[node_id].get_ip6_address(config.ADDRESS_TYPE.ALOC):