To try out the Beta versions of TS Licensing you do not need to purchase CALs from Microsoft. Instead, you can install test license codes from Microsoft so that you can install Windows Server “Longhorn” beta CALs on a Windows Server “Longhorn” beta license server. To request such CALs, you can go to the
Terminal Server License Code Request site
and request your own unique code. To get more information on installing test Beta CALs, you could refer the TS Licensing Step-By-Step guide.
Here's the complete list of Longhorn Licensing articles: