[tests] support testing commissioning (#3441)

Add support for testing commissioning process, including:

1. Add DTLS, Thread Discovery and commissioning messages(JOIN_FIN.req etc) parsing;
2. Support parsing log from UART response, and construct decrypted messages;
3. Common commands for commissioning related verification;

tests/scripts/thread-cert/pcap.py

@@ -76,6 +76,7 @@ class PcapCodec(object):
             timestamp = self._get_timestamp()
         pkt = self.encode_frame(frame, *timestamp)
         self._pcap_file.write(pkt)
+        self._pcap_file.flush()
 
     def __del__(self):
         self._pcap_file.close()