ext/spl: Deprecate ArrayObject and ArrayIterator with objects (#19420)

RFC: https://wiki.php.net/rfc/deprecations_php_8_5#deprecate_arrayobject_and_arrayiterator_with_objects

This also moves tests into a subfolder.

ext/spl/tests/ArrayObject/arrayObject_offsetExists_nullcheck.phpt

@@ -0,0 +1,10 @@
+--TEST--
+SPL: ArrayObject::offsetExists() should return true for element containing NULL
+--FILE--
+<?php
+$ao = new ArrayObject(array('foo' => null));
+var_dump($ao->offsetExists('foo'));
+
+?>
+--EXPECT--
+bool(true)