WooCommerce Subscriptions Failed Payment Retry Rules for Testing: Replace the default WooCommerce Subscriptions failed payment retry rules with rules that run every few minutes (not days) and always send dunning emails to help with testing.

wcs-test-retry-rules.php

<?php
/**
 * Plugin Name: WooCommerce Subscriptions Failed Payment Retry Rules for Testing
 * Plugin URI:
 * Description: Replace the default WooCommerce Subscriptions failed payment retry rules with rules that run every few minutes (not days) and always send dunning emails to help with testing.
 * Author: Prospress Inc.
 * Author URI: http://prospress.com/
 * Version: 1.0
 *
 * Copyright 2016 Prospress, Inc.  (email : freedoms@prospress.com)
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * @package		WooCommerce Subscriptions
 * @author		Prospress Inc.
 * @since		1.0
 */

/**
 * Apply our own set of retry rules.
 */
function wcsfptrr_retry_rules( $default_retry_class ) {
	return array(
		array(
			'retry_after_interval'            =>  60,
			'email_template_customer'         => 'WCS_Email_Customer_Payment_Retry',
			'email_template_admin'            => 'WCS_Email_Payment_Retry',
			'status_to_apply_to_order'        => 'pending',
			'status_to_apply_to_subscription' => 'on-hold',
		),
		array(
			'retry_after_interval'            => 120,
			'email_template_customer'         => 'WCS_Email_Customer_Payment_Retry',
			'email_template_admin'            => 'WCS_Email_Payment_Retry',
			'status_to_apply_to_order'        => 'pending',
			'status_to_apply_to_subscription' => 'on-hold',
		),
		array(
			'retry_after_interval'            => 180,
			'email_template_customer'         => 'WCS_Email_Customer_Payment_Retry',
			'email_template_admin'            => 'WCS_Email_Payment_Retry',
			'status_to_apply_to_order'        => 'pending',
			'status_to_apply_to_subscription' => 'on-hold',
		),
		array(
			'retry_after_interval'            => 240,
			'email_template_customer'         => 'WCS_Email_Customer_Payment_Retry',
			'email_template_admin'            => 'WCS_Email_Payment_Retry',
			'status_to_apply_to_order'        => 'pending',
			'status_to_apply_to_subscription' => 'on-hold',
		),
		array(
			'retry_after_interval'            => 300,
			'email_template_customer'         => 'WCS_Email_Customer_Payment_Retry',
			'email_template_admin'            => 'WCS_Email_Payment_Retry',
			'status_to_apply_to_order'        => 'pending',
			'status_to_apply_to_subscription' => 'on-hold',
		),
	);
}
add_filter( 'wcs_default_retry_rules', 'wcsfptrr_retry_rules' );

In it's previous form, tested 23rd Dec 2016, the plugin caused a fatal error. Updating add_filter() as above resolved the issue.
The version shown in WooCommerce dev docs differs but is also problematic for a similar reason.

Thanks for the plugin :)