skip test if Xdebug not loaded

Author: staabm

tests/end-to-end/regression/6105.phpt

@@ -1,5 +1,9 @@
 --TEST--
 https://github.com/sebastianbergmann/phpunit/issues/6105
+--SKIPIF--
+<?php if(!extension_loaded('xdebug')) {
+ print 'skip: xdebug is not loaded';
+}
 --FILE--
 <?php declare(strict_types=1);
 $_SERVER['argv'][] = '--do-not-cache-result';