Add defined port fallback for network echo tests

The 0 aka 'any' port doesn't for on Mac, but, only, using defined port
may interfere with running multiple tests on CI.

Author: bulislaw

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

@@ -128,7 +128,10 @@ def find_interface_to_target_addr(target_ip):
         :return:
         """
         s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-        s.connect((target_ip, 8000)) # Target IP, 'random' port
+        try:
+            s.connect((target_ip, 0)) # Target IP, any port
+        except socket.error:
+            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 +55,10 @@ def find_interface_to_target_addr(target_ip):
         :return:
         """
         s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
-        s.connect((target_ip, 8000)) # Target IP, 'random' port
+        try:
+            s.connect((target_ip, 0)) # Target IP, any port
+        except socket.error:
+            s.connect((target_ip, 8000)) # Target IP, 'random' port
         ip = s.getsockname()[0]
         s.close()
         return ip