If you want to test the whole of your application, then your application must be able to accept input from the test-code and return results which the test-code can ... test. Whether that will be possible depends entirely upon your application.
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James
My blog: Imperial Deltronics