tests: Use .errno instead of .args[0] for OSError exceptions.

Signed-off-by: Damien George <damien@micropython.org>

tests/multi_net/tcp_client_rst.py

@@ -33,7 +33,7 @@ def instance0():
         print(s2.recv(10))
         print(convert_poll_list(poll.poll(1000)))
     except OSError as er:
-        print(er.args[0])
+        print(er.errno)
     print(convert_poll_list(poll.poll(1000)))
     # TODO lwip raises here but apparently it shouldn't
     print(s2.recv(10))