When testing emails, an email administrator should be able to see what will populate in the merge fields without having to click "send final" for a test email with a test list and create a new email and "send final" again with the final recipient list. One way to do this would be to create a constituent data source list for a test group, require it to be called in the test list field, and as long as those two requirements are met, when you click "send test" instead of "send final" it will populate merge fields for the testers. I can understand if you're just putting standalone email addresses in the test email that it would populate information in the merge fields, but it seems inefficient to require an email administrator to create two separate emails to use "send final" with - one for test lists and one for recipient lists.