do you want to test if the servo is centered exactly or if the servo will rotate continuously.
If you want to test if the servo is centered correctly just plug the servo in with the centering code loaded and the servo's motor should not move at all.
if you want to test if the servo is centered correctly the photovore code will work as it works without a sensor. Just build the .hex file like mentioned in the $50 robot tutorial and load it to the MCU and plug in a servo and connect the power. your servo should move smoothly. If nothing happens hold the servo to your ear and if you can hear a buzzing noise something is wrong with the way you modified the servo.