Kohana Unittest module has made testing in Kohana v3 easy and fun. Lysender wrote a cool, straight forward post about how to set it up and running. I'll just talk here about how I have set up database testing with Kohana Unittest for my personal use. Not saying "How to set up". Because it's obviously not the best way to do this.
They walk you through the creation of the testing process - making "setup" and "tear down" SQL files, creating a "runSchema" method in a test case class and the code needed to run those setup/tear down files to refresh the database so there's no side effects of multiple tests using the same database.