Run gh24173_write_timeout.phpt on Windows only

The test still hit the run-tests timeout on macOS, although the fix is
Windows only, so timed writes to a stalled loopback peer don't time out
reliably there either way. The bug and the fix are Windows specific, so
limit the test to Windows like gh24173.phpt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: vibbow

ext/standard/tests/streams/gh24173_write_timeout.phpt

@@ -1,5 +1,9 @@
 --TEST--
 GH-24173 (Timed writes on socket streams must not block past the timeout)
+--SKIPIF--
+<?php
+if (PHP_OS_FAMILY !== 'Windows') die('skip Windows only');
+?>
 --FILE--
 <?php
 $srv = stream_socket_server('tcp://127.0.0.1:0', $errno, $errstr);