Add test for asort stability

Author: nikic

ext/standard/tests/array/asort_stability.phpt

@@ -0,0 +1,12 @@
+--TEST--
+asort() is stable
+--FILE--
+<?php
+
+$array = $origArray = array_fill(0, 1000, null);
+asort($array);
+var_dump($array === $origArray);
+
+?>
+--EXPECT--
+bool(true)