<?php
/*
    Template Name: Random Post
*/

remove_action( 'genesis_loop', 'genesis_do_loop' );
add_action( 'genesis_loop', 'prefix_do_loop' );

function prefix_do_loop() {

    global $paged;

    $args = (array(
        'orderby'       => 'rand',
        'paged'          => $paged,
        'posts_per_page' => 1,
        'category_name' => 'testimonials'
    ));

    genesis_custom_loop( $args );
}

remove_action( 'genesis_after_endwhile', 'genesis_posts_nav' );

genesis();