Code Modernization: Remove dynamic properties in `WP_Test_REST_Posts_Controller`.

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 particular case, the `test_create_update_post_with_featured_media()` method creates an attachment and writes the ID of the attachment to a dynamic (undeclared) property to be used as a flag to determine whether attachments need to be cleaned up after the test in the `tear_down()` method.

As the actual ''value'' of the property is irrelevant for the cleaning up and the property is realistically being used as a “flag”, this is now fixed as follows:

* Have an actual “flag” property declared with a descriptive name — `$attachments_created` — to make the code in the `tear_down()` more easily understandable.
* As for the actual ID of the attachment, save that to a test method local variable as that is the only place where it has any relevance.

Includes moving the `tear_down()` method up to be directly below the `set_up()` method.

Follow-up to [38832], [53557], [53558], [53850], [53851], [53852], [53853], [53854], [53856], [53916].

Props jrf.
See #56033.
Built from https://develop.svn.wordpress.org/trunk@53935


git-svn-id: http://core.svn.wordpress.org/trunk@53494 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53934';
+$wp_version = '6.1-alpha-53935';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.