test email - can I change the name from 'test' to 'preview'?

No you cannot change that since you are sending "sample".  You can use a batch campaign or email program to send out the email.  However, theses activities will be tracked in the lead's activity history.

This would be a nice thing to change, or have some kind of "live test" that could be sent. I know Litmus' spam check gets upset about having the word "TEST" in subject lines.

Thanks Matt, I agree! There are other words than test that would convey it isn't the official deployment without flagging spam checks and worrying people on a seed list. I would love to see Marketo change this to Preview, or allow each instance to choose their own prefix.