Test. You can find the API docs at https://na24.lightning.force.com/auradocs/reference.app#reference?topic=api:Test.
The docs are...sparse to say the least. The documentation for the Aura open source framework seems to say you can create a MyComponentTest.js file in your component bundle with tests, but no tool I can find successfully deploys a test class to Salesforce.
Is there a way to write test classes into my Controller that are called when certain events fire? Or are we stuck waiting for Salesforce to support this aspect of Aura?