I would recommend not depending on a fixed set of data, but rather having your tests setup the data they're going to work with, and clean up after themselves. Forcing them to do the setup means you'll be inclined to write your main code such that that's easy, and when you need to change your test data it's right alongside your test code. The cleanup part can be a little trickier, mainly because you may or not want to cleanup when you fail (if you want to inspect the data at the time of failure, you'll want to leave it, but most of the time you won't need it and you'll want it cleaned up for you).