Tests: Correct magic methods in Basic_Object.

This is a test fixture (dummy class only used in a test context), which incorrectly implements the magic methods.

With the deprecation of dynamic properties in PHP 8.2, this needs to be fixed.

The new implementation represents a “proper” implementation of the magic methods for a class without non-`public` or typed properties.

Notes:

* Instead of relying on dynamic properties, the magic methods now store properties in a `private` `$arbitrary_props` array and retrieve them from there as well.
* The original `$foo` property, even though declared as `private`, was never `private` in practice due to the way the magic methods were originally implemented. In effect, it was fully publicly retrievable and modifiable without any (type) restrictions. With that in mind, the `foo` property has been moved into the `$arbitrary_props` array to keep the implementation of the magic methods as clean and straightforward as possible. With the adjusted magic methods, access to and modification of `$foo` will (on the surface) continue to work in the same way as before, while under the hood, it is no longer affected by the dynamic properties deprecation.
* Take note of the use of `array_key_exists()` instead of `isset()` in the `__get()` method. This is intentional and allows for `null` values to be stored and retrieved.
*  Also take note of `__set()` method no longer returning. `__set()` is supposed to be a `void` method. In practice, the return value would always be ignored due to how PHP handles magic methods, so in effect, this change will not make any difference and does not constitute a backward compatibility break.[[BR]][[BR]]
 > The return value of `__set()` is ignored because of the way PHP processes the assignment operator.

Alternatives considered:

* Instead of fixing the magic methods, they could have been removed instead and the class be made to `extend` `stdClass`. It has been chosen not to do so for two reasons:
 1. It’s kind of nice to have at least ''one'' correct implementation of magic methods in WP, which can be used as an example to point to as well.
 2. Extending `stdClass` would change the class hierarchy, which ''may'' or ''may not'' affect the tests using this fixture (depending on what’s being done with the class). Extending `stdClass` would also obfuscate what’s going on in the class and would require extensive documentation to prevent the extension being inadvertently removed at a future point in time.
* Instead of fixing the magic methods, the test fixture could have been deprecated and/or removed, with the few tests which use the fixture being updated to use `stdClass` for their test fixture instead. It has been chosen not to do so as there may well be external (plugin/theme) tests relying on this test fixture and evaluating whether that is so would be hard, as WP Directory cannot be used, since test code is normally not included in the code published on wp.org. Also note, there is still a (deprecated) `Basic_Subclass` fixture in the test suite, which extends this class.

These magic methods and the `Basic_Object` test fixture were originally introduced in [28480] and [28523]. The fixture was deprecated in [42381] and undeprecated again in [45807].

At this time, the test fixture is used in the `WP_Test_REST_Post_Meta_Fields` and the `Tests_REST_API` test classes.

References:
* [https://www.php.net/manual/en/language.oop5.overloading.php#object.set PHP Manual: Overloading: __set()]
* [https://wiki.php.net/rfc/deprecate_dynamic_properties PHP RFC: Deprecate dynamic properties]
* [php/php-src#7786 php-src: #7786 PHP 8.2: unexpected deprecation for dynamic property set via magic method]

Follow-up to [28480], [28493], [28523], [42381], [45807].

Props jrf, costdev.
See #56514.
Built from https://develop.svn.wordpress.org/trunk@54095

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-54094';
+$wp_version = '6.1-alpha-54095';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.