wp_die_handler() in the test suite should accept a WP_Error object

[johnbillion]

The default handlers for calls to wp_die() accept a WP_Error object in the $message parameter. The handler in the test suite does not.

Core passes a WP_Error object to wp_die() in a few places, for example in wp-includes/template-loader.php if there's a problem with the theme. Currently, this error object gets converted to a string in the test suite, resulting in a test failure message that states WPDieException: 0.

[johnbillion]

In 44944:

Build/Test tools: Add support for passing a WP_Error object to wp_die() during tests.

This brings the functionality inline with wp_die() during non-test usage.

Fixes #46460