lwip: Added delay to dtls handshake test to compensate for local network

The speed of packets on the local network exceeds even the speed of
the ethernet hardware on some of the less powerful devices. Adding
a small delay which can be expected from a real DTLS handshake prevents
this condition from occuring.

features/FEATURE_LWIP/TESTS/mbedmicro-net/host_tests/udp_shotgun.py

@@ -20,6 +20,7 @@ import socket
 import json
 import random
 import itertools
+import time
 from sys import stdout
 from threading import Thread
 from SocketServer import BaseRequestHandler, UDPServer
@@ -42,6 +43,9 @@ class UDPEchoClientHandler(BaseRequestHandler):
             data = ''.join(map(chr, data))
             sock.sendto(data, self.client_address)
 
+            # Sleep a tiny bit to compensate for local network
+            time.sleep(0.01)
+
 
 class UDPEchoClientTest(BaseHostTest):
     def __init__(self):