[skip ci] Mark gh17399_substr.phpt as slow

Author: iluuu1994

ext/iconv/tests/gh17399_substr.phpt

@@ -4,6 +4,10 @@ GH-17399 (iconv_substr() leak on bailout)
 iconv
 --INI--
 memory_limit=2M
+--SKIPIF--
+<?php
+if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
+?>
 --FILE--
 <?php
 $str = str_repeat('a', 1024 * 1024);