Help with programming a tester!

[johnwasser]

@eopett

if ( pressDuration = constrainedInput ) {

First of all, you probably meant:
if ( pressDuration == constrainedInput ) {

Second, what are the chances that the value will be exactly the same as your serial input?

@eopett

int input = Serial.parseInt();  // keep other operations outside the constrain function

You can't call Serial.parseInt() until AFTER you call Serial.begin(baudrate);. Move the input = Serial.parseInt(); to setup(). What is the serial input for? You already know the allowed range of values.