To make an unit test in VFP is as easy as anything: just write your code, highlight it, activate the shortcut (right mouse click), select the option 'Execute the selection'. A very neat and quick solution.
It's even quicker if you use FoxUnit and foxmock. Both are free on VFPX. I've hundreds of unit tests that I can run with a single key stroke that test many different modules of our applications. Writing testable code also tends to lead to a cleaner class design that is more reusable.