exclude custom post type from search

// functions.php

add_action( 'init', 'update_testimonials', 99 );

function update_testimonials() {
	global $wp_post_types;

	if ( post_type_exists( 'testimonials' ) ) {

		// exclude from search results
		$wp_post_types['testimonials']->exclude_from_search = true;
	}
}