Experience at LMAX has very clearly demonstrated the benefits of good test isolation, so one of the first things I added to Moolah was multi-tenant support. Each account is a completely isolated little world which is perfect for testing.
Given that I’ve completely punted on storing passwords by delegating authentication to Google (and potentially other places in the future since it’s using bell to handle third party authentication), there’s actually no user creation step at all which makes it even easier. As a result it’s not just acceptance tests that are benefiting from the isolation but also the database tests which no longer need to delete everything in the database to give themselves a clear workspace.
It’s all quite civilised really.