When you say test case, using ATF or specifically test a flow?
There is inbuilt Test capabilities in Flow Designer so you can test Flows are you design them, or want to see what they are doing.
For ATF, the focus should not be testing the specific components of the Flow, ATF's purpose is testing the process. So for example if you have an ATF where a Flow assigns a record to a group, in your ATF just set the conditions and validate afterwards the group assigned is correctly. So your testing the inputs and validate the purpose of the Flow.
And then as always remember ATF is just for automated the testing of the process. It's not about testing every single field and component, its about making sure a process is running correctly.
