Tests: Add coverage for WP_Fatal_Error_Handler.
Cover error detection, handled error types, early returns, and default error messages and filters. Isolate tests that depend on sandbox or header state, explicitly establish their preconditions, and verify the error details and response arguments passed to wp_die().
Developed in: https://github.com/WordPress/wordpress-develop/pull/14163
Props huzaifaalmesbah.
See #65819.
Javascript required