Build/Test Tools: Add support for `WP_Error` in the test suite's `wp_die()` handlers.

This brings parity with WordPress core `wp_die()` handlers and ensures that if a `WP_Error` object is passed as the `$message` argument to `wp_die()`, the PHPUnit test suite displays the error message correctly.

Previously, this would cause a silent fatal error: `Object of class WP_Error could not be converted to string`, leading to just displaying `wp_die called` without any further details.

Follow-up to [28797], [41966], [44666], [45160], [47882].

See #55652.
Built from https://develop.svn.wordpress.org/trunk@53634


git-svn-id: http://core.svn.wordpress.org/trunk@53193 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53633';
+$wp_version = '6.1-alpha-53634';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.