tests/extmod: Close UDP sockets at end of test.

This adds call to release UDP port in a timely manner, so they can be
reused in subsequent tests.  Otherwise, one could face issue like #17623.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

tests/extmod/select_poll_udp.py

@@ -29,3 +29,5 @@ print(poll.poll(0)[0][1] == select.POLLOUT)
 if hasattr(select, "select"):
     r, w, e = select.select([s], [], [], 0)
     assert not r and not w and not e
+
+s.close()

tests/extmod/socket_udp_nonblock.py

@@ -19,3 +19,5 @@ try:
     s.recv(1)
 except OSError as er:
     print("EAGAIN:", er.errno == errno.EAGAIN)
+
+s.close()