Code Modernization: Remove dynamic properties in `Tests_Comment_Walker`.

Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.

In this test file, the `Tests_Comment_Walker::set_up()` method created a dynamic `$post_id` property, which should have been explicitly declared.

Along the same lines, the `Comment_Callback_Test::__construct()` method also assigned values to two undeclared properties.

This is now fixed by explicitly declaring the properties in both classes.

Includes renaming the `Comment_Callback_Test` class to `Comment_Callback_Test_Helper` as it does not contain any test methods and its only purpose is as a “helper” class for the `Tests_Comment_Walker::test_has_children()` test.

Follow-up to [28824], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916], [53935], [53936], [53937].

Props jrf.
See #56033.
Built from https://develop.svn.wordpress.org/trunk@53938


git-svn-id: http://core.svn.wordpress.org/trunk@53497 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53937';
+$wp_version = '6.1-alpha-53938';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.