This feature request originally appeared in the Questions section of our website. I moved it to the Suggestions area so that we can gather insight on how many customers would like to be able to send test autoresponse emails.
Additionally, as Ana Escobedo noted below, payment processors usually provide test account numbers to allow you to complete test donations. Please check with your payment processor for a complete list. For example, PayPal provides a list of test credit card account numbers.
You can see what the default donation receipt autoresponder looks like in our donation page HOWTO.