Code Modernization: Remove dynamic properties in `Tests_Media_GetPostGalleries`.

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 class contains a `set_up()` method that sets the `$img_meta` property, which is ''used'' by the tests, but never ''changed'' by the tests.

In other words, setting this property in the `set_up()` is an unnecessary overhead and the property should be changed to a class constant.

Includes renaming the test class to match the [https://make.wordpress.org/core/handbook/testing/automated-testing/writing-phpunit-tests/#naming-and-organization naming conventions].

Follow-up to [52190], [53557], [53558], [53850], [53851].

Props jrf.
See #56033.
Built from https://develop.svn.wordpress.org/trunk@53852


git-svn-id: http://core.svn.wordpress.org/trunk@53411 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53851';
+$wp_version = '6.1-alpha-53852';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.