Generate the post slug for the draft testimonial (CPT) as well

functions.php

/**
 * Generate the post slug for the draft post as well
 *
 * @param int     $post_id Post ID.
 * @param WP_Post $post    Post object.
 * @param bool    $update  Whether this is an existing post being updated.
 **/
function lubus_generate_slug_for_draft_post( $post_id, $post, $update ) {

	// If the post is a draft and has no slug.
	if (
		'draft' === $post->post_status
		&&
		( empty( $post->post_name )
		|| 'auto-draft' == $post->post_name )
	) {

		// Generate a slug from the post title.
		$slug = sanitize_title( $post->post_title );

		// Remove the action to avoid infinite loop.
		remove_action( 'save_post', 'lubus_generate_slug_for_draft_post', 10000 );

		// Update the post with the new slug.
		wp_update_post(
			array(
				'ID'        => $post_id,
				'post_name' => $slug,
			)
		);

		// Re-add the action.
		add_action( 'save_post', 'lubus_generate_slug_for_draft_post', 10000, 3 );
	}
}
add_action( 'save_post_testimonial', 'lubus_generate_slug_for_draft_post', 10000, 3 );