Add a XFAIL test for #64896

Zend/tests/bug64896.phpt

@@ -0,0 +1,47 @@
+--TEST--
+Bug #64896 (Segfault with gc_collect_cycles using unserialize on certain objects)
+--XFAIL--
+We can not fix this bug with a significant (performace slow down) change to gc
+--FILE--
+<?php
+$bar = NULL;
+class bad
+{
+	private $_private = array();
+
+	public function __construct()
+	{
+		$this->_private[] = 'php';
+	}
+
+	public function __destruct()
+	{
+		global $bar;
+		$bar = $this;
+	}
+}
+
+$foo = new stdclass;
+$foo->foo = $foo;
+$foo->bad = new bad;
+
+gc_disable();
+
+unserialize(serialize($foo));
+gc_collect_cycles();
+var_dump($bar); 
+/*  will output:
+object(bad)#4 (1) {
+  ["_private":"bad":private]=>
+  &UNKNOWN:0
+}
+*/
+?>
+--EXPECTF--
+bject(bad)#%d (1) {
+  ["_private":"bad":private]=>
+  array(1) {
+    [0]=>
+    string(3) "php"
+  }
+}