Me thinks we have had many Test Servers since launch. The entire game, really began a new phase of beta testing on Jan 26th.
Now, don't get me wrong here. I think a dedicated Test Server has merit. But for the life of me I don't see how you will attract folks to the Test Server, when they are testing the game daily on the live servers.
I think you are going to have to do more than wave the "be the first to test new content carrot" in front of the players. At the very least, directives, anticipated new content to test, and so on would be in order.
Finally, make it EASY to get onto test. Right now, everyone is doing hoola hoops, renaming directories, downloading unnecessary files, and so on. It just makes for a very frustrating experience for the user, and most will not have the patience to sort through the muck to get on test.
Just my coppers.