Create Custom URL structure for specific Post category

Try these steps:

Step #1: Replace this:

add_action('generate_rewrite_rules', 'custom_rewrite_rules');
function custom_rewrite_rules( $wp_rewrite ) {
    $regex = '/[\s\S]/';
    $new_rules = array(
      trailingslashit('testimonials/'.$regex) => '/?p='. get_page_by_path('$matches[1]', OBJECT, 'post')->id
    );
    $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
    return $wp_rewrite;
}

..with this one:

add_action( 'init', 'custom_rewrite_rules' );
function custom_rewrite_rules() {
    add_rewrite_rule(
        'testimonials/([^/]+)(?:/([0-9]+))?/?$',
        'index.php?category_name=testimonials&name=$matches[1]&page=$matches[2]',
        'top' // The rule position; either 'top' or 'bottom' (default).
    );
}

Step #2: Go to the Permalink Settings page, and click on the Save Changes button without actually making any changes.