Fix test so it still acts as a regression test

Author: Girgias

ext/standard/tests/filters/gh17037.phpt

@@ -2,11 +2,13 @@
 GH-17037 (UAF in user filter when adding existing filter name due to incorrect error handling)
 --FILE--
 <?php
+
+class filter_string_toupper {}
 try {
 	var_dump(stream_filter_register('string.toupper', 'filter_string_toupper'));
 } catch (Throwable $e) {
     echo $e::class, ': ', $e->getMessage(), PHP_EOL;
 }
 ?>
 --EXPECT--
-TypeError: stream_filter_register(): Argument #2 ($class) must be a valid class name, filter_string_toupper given
+bool(false)