Wordpress stores the URL it uses in several places in the database. You need to change the URL in the database and change it back when you want to use the original domain. However, normally the temp URL will not work with PHP scripts because we run it as suPHP. You may see an Internal Server Error.
A simpler way to test without changing the URLs is to use your hosts file to switch the address for your domain for yourself only. See
http://www.lunarforums.com/forum/index.php?topic=20444.msg150792#msg150792