It would be useful to be able to include a note for reviewers that gets inserted above or below the actual email when doing a test send. It should be something short, perhaps 250 characters or so and highlighted in some way that's its a test note and would not appear in the live send. 

Many times before sending a test it's necessary to send a separate (personal) email to all the reviewers letting them know the test is coming, what its for and when any edits or comments are due back. Being able to include a note within the test with this information would eliminate the need of sending a separate email with those notes.