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_modify_shared_object_properties.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Modifications to ArrayObjects should not affect shared properties tables
+--FILE--
+<?php
+
+$obj = (object)['a' => 1, 'b' => 2];
+$ao = new ArrayObject($obj);
+$arr = (array) $obj;
+$ao['a'] = 42;
+var_dump($arr);
+
+?>
+--EXPECTF--
+Deprecated: ArrayObject::__construct(): Using an object as a backing array for ArrayObject is deprecated, as it allows violating class constraints and invariants in %s on line %d
+array(2) {
+  ["a"]=>
+  int(1)
+  ["b"]=>
+  int(2)
+}