[tests] add traffic analysis for Cert_5_3_08 as TestPlan (#2484)

This commit adds the method set_lowpan_context to set the lowpan context of sniffer.

tests/scripts/thread-cert/node_cli.py

@@ -32,6 +32,7 @@ import sys
 import time
 import pexpect
 import re
+import ipaddress
 
 import config
 
@@ -388,6 +389,17 @@ class otCli:
 
         return addrs
 
+    def get_addr(self, prefix):
+        network = ipaddress.ip_network(unicode(prefix))
+        addrs = self.get_addrs()
+
+        for addr in addrs:
+            ipv6_address = ipaddress.ip_address(addr.decode("utf-8"))
+            if ipv6_address in network:
+                return ipv6_address.exploded.encode('utf-8')
+
+        return None
+
     def add_service(self, enterpriseNumber, serviceData, serverData):
         cmd = 'service add ' + enterpriseNumber + ' ' + serviceData+ ' '  + serverData
         self.send_command(cmd)