Custom post type – how can I make it a sub-url of another page?

When you register the CPT, use the rewrite argument to add the prefix.

For example if your CPT is currently registered like this:

<?php
function wpse_create_cpt_testimonial() {
    $args = array(
        'public' => true,
        'label'  => __( 'Testimonials', 'textdomain' ),
        // ... you probably have other arguments as well
    );
    register_post_type( 'testimonial', $args );
}
add_action( 'init', 'wpse_create_cpt_testimonial' );
?>

you can add rewrite like this:

<?php
function wpse_create_cpt_testimonial() {
    $args = array(
        'public' => true,
        'label'  => __( 'Testimonials', 'textdomain' ),
        // Here's the part where you're changing the URL
        'rewrite' => array('slug' => 'results/testimonials'),
        // If you want that URL to be an archive, add this line too
        'has_archive' => 'results/testimonials',
        // ... keep your other arguments as well
    );
    register_post_type( 'testimonial', $args );
}
add_action( 'init', 'wpse_create_cpt_testimonial' );
?>

If this doesn’t change things immediately, you may need to run unregister_post_type('testimonial') right before re-registering it, and/or visit the Permalinks page to flush rewrite rules.