Tests: Bring some consistency to creating and updating objects in factory classes.

In various unit test factory classes, some of the `create_object()` and `update_object()` methods returned a `WP_Error` object on failure, while a few others were documented to do so, but did not in practice, instead returning the value `0` or `false`, or not accounting for a failure at all.

This commit aims to handle this in a consistent way by updating the methods to always return the object ID on success and a `WP_Error` object on failure.

Includes:
* Updating and correcting the relevant documentation parts.
* Adding missing documentation and `@since` tags in some classes.
* Renaming some variables to clarify that it is the object ID which is passed around, not the object itself.

Follow-up to [760/tests], [838/tests], [922/tests], [948/tests], [985/tests], [27178], [32659], [34855], [37563], [40968], [44497], [46262].

See #56793.
Built from https://develop.svn.wordpress.org/trunk@55019


git-svn-id: http://core.svn.wordpress.org/trunk@54552 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55018';
+$wp_version = '6.2-alpha-55019';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.