Agree, having to wait one hour to test the notification is a problem. Changing this is already contemplated in the notifications migration.
For now, as a workaround, maybe you could use a comments and attachment notification to test the email body. Since it triggers right away, you could add a comment and flush the queue; that way, you don’t have to wait one hour.
Of course, this is just a workaround for the moment, we are really looking into adding some actual buttons to test the notification or at least a way to see what the actual email body would look like.