The functional testing you do for D7 is of the HCI and use cases. The goal is to verify that your use cases and screens meet the users requirements. You should learn from this testing that there are more and/or different requirements, you need due to how they wish you to test their proposed software. Testing is done again in design, again in construction, and again when deployment happens. Testing is one of those items that can DRASTICALLY slow down a project. If you don't test well it only gets slower.