Fix GH-15331: dom: Element::$substitutedNodeValue test failed

Bug in libxml, amend test to accept both outputs.

NEWS

@@ -24,6 +24,7 @@ PHP                                                                        NEWS
     (html5_serializer)). (nielsdos)
   . Deprecated DOM_PHP_ERR constant. (nielsdos)
   . Removed DOMImplementation::getFeature(). (nielsdos)
+  . Fixed bug GH-15331 (Element::$substitutedNodeValue test failed). (nielsdos)
 
 - Hash:
   . Deprecated passing incorrect data types for options to ext/hash functions.

ext/dom/tests/modern/extensions/Element_substitutedNodeValue.phpt

@@ -23,7 +23,7 @@ var_dump($dom->documentElement->nodeValue); // Should always be NULL for element
 echo $dom->saveXML(), "\n";
 
 ?>
---EXPECT--
+--EXPECTF--
 string(1) "1"
 NULL
 <?xml version="1.0" encoding="UTF-8"?>
@@ -35,4 +35,4 @@ NULL
 string(0) ""
 NULL
 <?xml version="1.0" encoding="UTF-8"?>
-<root></root>
+%r(<root><\/root>|<root\/>)%r