test-notice.php

<?php
/*
Plugin Name: Add Test Notice
Description: Adds a test notice.
Author: WooCommerce
Version: 1.0
*/

function test_add_notice() { ?>
	<div class="notice notice-warning is-dismissible">
		<p>
			<strong>Testing notice.</strong>
		</p>
	</div>
<?php }

add_action( 'admin_notices', 'test_add_notice' );