Code Modernization: Silence the deprecation warning for missing return type in `phpunit/tests/compat.php`.

This fixes the "Deprecated: Return type of `CountableFake::count()` should be compatible with `Countable::count(): int`" warning on PHP 8.1.

PHP native interfaces now have declared return types and methods in classes implementing these interfaces need to either have the return type declared (in a covariant compatible manner with the PHP native interface method declaration), or need to silence the deprecation warning using the `#[ReturnTypeWillChange]` attribute.

Follow-up to [51517], [51529], [51530], [51531], [51532].

Props jrf.
See #53635.
Built from https://develop.svn.wordpress.org/trunk@51594


git-svn-id: http://core.svn.wordpress.org/trunk@51205 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51592';
+$wp_version = '5.9-alpha-51594';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.