Tests need to cover small units one at a time. I've been bitten too many times by tests that were too complex and attempted to test too many things at once. The

Tests need to cover small units one at a time. I've been bitten too many times by tests that were too complex and attempted to test too many things at once. They invariably ended up doing nothing of value. Test setup becomes overly complex and prone to oversights. Writing another test for a variation of a problem that takes a slightly different code path becomes a chore. It also gets hard to spot that odd untested corner case hidden deep inside some complex logic. The proper way to think about this is to have unit tests that cover the nooks and crannies of the business logic and integration tests that make sure that interfaces are correct.