Blizzard has a Tournament Test Realm open for everyone to log on and play with. The test realm let them get the mechanics for the real tournament realm ready before hand, allowing Blizzard to work all the kinks out.
We had a lot of coverage of the event, and some of it is pretty interesting to go back and read.
There were two main tests, called stress tests. These were where as many people as possible logged on and attempted to play. You can check out our coverage of Stress Test I and Stress Test II.
And of course, the highlights of the stress test were the GMs that came and spawned all sorts of terrible creations.