If you mean the wrapper that would temporarily substitute all the recipient's (customers) addresses in outgoing mails with an arbitrary address specified by the admin, actually we already can test the outgoing mail with a single (test) customer operation, without changing any configuration system wide.
If there were issues sending per our email configuration, we get the error in the panel and error log file, currently.
What is the purpose of such a test mode per session ?
We thought the user was interested in the real-time monitoring, sending a test email to a specified external address at a fixed interval (eg daily).
She could achieve this without the feature/extension, by planting a script on her server that would be using the same email configuration as in her cart.
But then again, if Joel's concern includes public servers like Google, the Google would be the first mail server that would consider such same-text repetitive mails as spam if it keeps hitting your test gmail address.