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

tests/scripts/thread-cert/Cert_5_3_09_AddressQuery.py

@@ -86,8 +86,8 @@ class Cert_5_3_09_AddressQuery(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 test(self):
         # 1 & 2 ALL: Build and verify the topology