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_proptable_canonicalization.phpt

@@ -0,0 +1,34 @@
+--TEST--
+When ArrayObject wraps an object, we should use prop table canonicalization
+--FILE--
+<?php
+
+$o = new stdClass;
+$ao = new ArrayObject($o);
+$ao[0] = 1;
+var_dump($o);
+$ao[0] += 1;
+var_dump($o);
+var_dump(isset($ao[0]));
+var_dump((array) $ao);
+unset($ao[0]);
+var_dump($o);
+
+?>
+--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
+object(stdClass)#1 (1) {
+  ["0"]=>
+  int(1)
+}
+object(stdClass)#1 (1) {
+  ["0"]=>
+  int(2)
+}
+bool(true)
+array(1) {
+  [0]=>
+  int(2)
+}
+object(stdClass)#1 (0) {
+}