plugin.php

<?php
/**
 * Plugin Name: My Plugin 21689
 */

class My_Plugin_Test_21689 {
	function __construct() {
		add_action( 'init', array( $this, 'init' ) );
	}

	function init() {
		add_shortcode( 'test21689', array( $this, 'shortcode_callback' ) );
		echo apply_filters( 'the_content', "[test21689]\n\nsome content\n\n[/test21689]" );
		die();
	}

	function shortcode_callback( $args, $content ) {
		return $content;
	}
}
new My_Plugin_Test_21689;