Create an ACF block

custom-callback.php

function my_acf_block_render_callback( $block ) {
  $slug = str_replace('acf/', '', $block['name']);
  if( file_exists(STYLESHEETPATH . "/template-parts/block/content-{$slug}.php") ) {
    include( STYLESHEETPATH . "/template-parts/block/content-{$slug}.php" );
  }
}

register-block-testimonial.php

add_action('acf/init', function() {
  if( function_exists('acf_register_block') ) {
    acf_register_block(array(
      'name'        => 'testimonial',
      'title'        => __('Testimonial'),
      'render_callback'  => 'my_acf_block_render_callback',
      'category'      => 'formatting',
    ));
  }
}