If you look at the top of setup.php you’ll see

<?php

namespace App;

?>

This means that setup.php is in the App namespace. Whenever you pass callbacks to add_action or add_filter or whatever you must include the namespace. You can write the whole thing out, but in this context the easiest way is to use the __NAMESPACE__ magic constant. So something like this:

<?php

if( function_exists('acf_register_block_type') ) {
    add_action('acf/init', __NAMESPACE__.'\\register_acf_block_types');
}

?>
If you don’t anticipate needing to un-hook that action later, you can do what many of us do and pass an anonymous function as a callback instead:

<?php

if( function_exists('acf_register_block_type') ) {
    add_action('acf/init', function() {

      // register a testimonial block.
      acf_register_block_type(array(
        'name'              => 'testimonial',
        'title'             => __('Testimonial'),
        'description'       => __('A custom testimonial block.'),
        'render_template'   => 'app/Blocks/Testimonial/testimonial.php',
        'category'          => 'formatting',
        'icon'              => 'admin-comments',
        'keywords'          => array( 'testimonial', 'quote' ),
      ));
    });
}

?>