Survival test officially begins with the 0.24 update. It is only available to people who pre-pay for the game at €9.95 for a few weeks but then Notch decides to make it free. This version includes more interactive mobs, allows users to save and load survival maps, and adds Creeper enemies:
You have to pre-purchase the game to be able to participate in the test. This test is not an accurate demonstration of how the final survival mode will be. It does, however, contain many of the gameplay elements that will be included in Survival Mode. I’m making it available for you both to get some feedback while I’m in Indonesia for three weeks, and to thank you for pre-purchasing the game.