[tests] add traffic to 4 test cases: from Cert_5_2_07 to Cert_5_3_03 (#2214)

- Add a method in message.py to verify if receive a ICMPv6 message.
- Add two methods in command.py to verify link request and link accept.

tests/scripts/thread-cert/message.py

@@ -377,6 +377,14 @@ class MessagesSet(object):
 
         return message
 
+    def contains_icmp_message(self):
+        for m in self.messages:
+            if m.type == MessageType.ICMP:
+                return True
+
+        return False
+
+
     def contains_mle_message(self, command_type):
         for m in self.messages:
             if m.type != MessageType.MLE: