While there is nothing in the way of an automated built in method, you could make your own beta style selector or button and wrap it in a conditional to display only to specific usergroup id#s, and insert it with a template mod then when you don't need it anymore simply delete it.
For normal registered members who should see it just add a usergroup called testers and add those people as a secondary UG. It wont stop someone who knows the custom style id from checking it out, but if you are planning on taking it live soon that really shouldn't matter since I am assuming your are testing it for that reason.
The other alternative is to setup a dupe of your current forum as Brogan said.