You are better of letting usability test come first.
Besides how are you going to test unit GUI? What would be the output? If somebody clicks something and gets what he "initially" was looking for? What should be considered an unaccepted behaviour ?
You will get tons of bullcrap to look through.
Usability is a big word, but simple usability test is hiring a mate of yours(preferably not someone you know, a friend of a friend) who you might consider to be in application 'target audience'. Putting on a camtasia/camstudio (desktop recording software, might be good to record his face). Give him some tasks on a piece of paper(or in person, sometimes paper is better cause you wont be interfering - more lifelike scenrio). And watch what he is doing!
If he requires assist then you see sections of attention for future developement.
Never try to influence the guy telling him stuff like "but look here is this button, I thought its more convinient this way".
You will get much better output of such test, rather than waste your time with computer-to-computer testing. GUI is an INTERFACE between human and computer. Unit testing gui is like trying to parse the book you have just written and seeing if its any good.
Sure it will eliminate spellcheck errors for instance, but its a very small thing -> compared to book's real objectives.