Add unit tests for wp_ajax_get_post_thumbnail_html and `wp_ajax_set…

[pbearne]

This PR adds unit tests for wp_ajax_set_post_thumbnail() and wp_ajax_get_post_thumbnail_html() in src/wp-admin/includes/ajax-actions.php. The tests cover setting and removing featured images using both traditional and JSON-based requests, as well as permission and nonce validation.

• Trac ticket: https://core.trac.wordpress.org/ticket/65252

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