Trac ticket 53165 - Test Custom Taxonomy added as a must-use plugin

t53165.php

<?php
// Location of this code: wp-content/mu-plugins/t53165.php.

namespace Test\t53165;

add_action( 'init', __NAMESPACE__ . '\register_test_taxonomy' );
function register_test_taxonomy() {
	$labels = array(
		'name'              => 'Tests',
		'singular_name'     => 'Test',
		'search_items'      => 'Search Tests',
		'all_items'         => 'All Tests',
		'parent_item'       => 'Parent Test',
		'parent_item_colon' => 'Parent Test:',
		'edit_item'         => 'Edit Test',
		'update_item'       => 'Update Test',
		'add_new_item'      => 'Add New Test',
		'new_item_name'     => 'New Test',
		'menu_name'         => 'Test',
	);
	$args   = array(
		'hierarchical'      => true, // make it hierarchical (like categories)
		'labels'            => $labels,
		'public'            => true,
		'show_in_rest'		  => true,
		'show_admin_column' => true,
		'rewrite'           => array( 'slug' => 'test' ),
	);
	register_taxonomy( 'test', array( 'post' ), $args );
}