[test] add traffic analysis for Cert_5_2_3_LeaderReject2Hops (#2142)

Also add a method last_mle_message to find the newest mle; Add a method assertAssignedRouterQuantity to confirm if Leader contains the Route64 TLV with 32 assigned router IDs.

tests/scripts/thread-cert/message.py

@@ -156,6 +156,14 @@ class Message(object):
         assert(self.mle.command.type == command_type)
 
     def assertMleMessageContainsTlv(self, tlv_class_type):
+        """To confirm if Mle message contains the TLV type.
+
+        Args:
+            tlv_class_type: tlv's type.
+
+        Returns:
+            mle.Route64: If contains the TLV, return it.
+        """
         if self.type != MessageType.MLE:
             raise ValueError("Invalid message type. Expected MLE message.")
 
@@ -166,6 +174,22 @@ class Message(object):
                 break
 
         assert(contains_tlv == True)
+        return tlv
+
+    def assertAssignedRouterQuantity(self, router_quantity):
+        """Confirm if Leader contains the Route64 TLV with router_quantity assigned Router IDs.
+
+        Args:
+            router_quantity: the quantity of router.
+        """
+        tlv = self.assertMleMessageContainsTlv(mle.Route64)
+        router_id_mask = tlv.router_id_mask
+
+        count = 0
+        for i in range(1, 65):
+            count += (router_id_mask & 1)
+            router_id_mask = (router_id_mask >> 1)
+        assert(count == router_quantity)
 
     def assertMleMessageDoesNotContainTlv(self, tlv_class_type):
         if self.type != MessageType.MLE:
@@ -294,8 +318,37 @@ class MessagesSet(object):
 
         return message
 
+    def last_mle_message(self, command_type, assert_enabled = True):
+        """Get the last Mle Message with specified type from existing capture.
+
+        Args:
+            command_type: the specified mle type.
+            assert_enabled: interrupt or not when get the mle.
+
+        Returns:
+            message.Message: the last Mle Message with specified type.
+        """
+        message = None
+        size = len(self.messages)
+
+        for i in range(size - 1, -1, -1):
+            m = self.messages[i]
+
+            if m.type != MessageType.MLE:
+                continue
+
+            #for command_type in command_types:
+            if m.mle.command.type == command_type:
+                message = m
+                break
+
+        if assert_enabled:
+            assert message is not None, "Could not find MleMessage with type: {}".format(command_type)
+
+        return message
+
     def next_mle_message(self, command_type, assert_enabled=True):
-        message = self.next_mle_message_of_one_of_command_types(command_type,)
+        message = self.next_mle_message_of_one_of_command_types(command_type)
 
         if assert_enabled:
             assert message is not None, "Could not find MleMessage of the type: {}".format(command_type)