Fix network echo test host scripts for Mac

It seems that the 0 aka 'any port' doesn't work well on Mac, causing
[Errno 49] Can't assign requested address errors.

Author: bulislaw

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

@@ -128,7 +128,7 @@ def find_interface_to_target_addr(target_ip):
         :return:
         """
         s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-        s.connect((target_ip, 0)) # Target IP, Any port
+        s.connect((target_ip, 8000)) # Target IP, 'random' port
         ip = s.getsockname()[0]
         s.close()
         return ip

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

@@ -55,7 +55,7 @@ def find_interface_to_target_addr(target_ip):
         :return:
         """
         s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-        s.connect((target_ip, 0)) # Target IP, Any port
+        s.connect((target_ip, 8000)) # Target IP, 'random' port
         ip = s.getsockname()[0]
         s.close()
         return ip