The more I become experienced with testing, the less I think the difference between functional, integration and unit tests is important. What's important is to have a test suite with good coverage that runs fast. I found that a mix of high level and lower level tests is a good way to achieve this goal and I don't mind mixing them in the same files. High level tests provide coverage more quickly, but some stuff, such as verifying that an exception is handled properly, is easier to test with unit tests.