Selecting a dynamic sidebar via metabox in wordpress

Your code seems a little superfluous; I’m pretty sure you could compact it to simply:

<?php

do_action( 'before_sidebar' );

if ( ! $sidebar = get_post_meta( get_the_ID(), '_cmb_test_select', true ) )
    $sidebar="sidebar-1"; // Default sidebar if meta value does not exist.

if ( ! dynamic_sidebar( $sidebar ) ) : ?>

    <!-- No widgets for selected sidebar, display any default content here. --> 

<?php endif ?>