It is called unit testing. You test the backlight all by itself, without initializing the graphics subsystem. You also have coverage metrics that identify all of the input states that can affect the backlight, including any that initializing the gpu may modify, and you test the backlight code against each possible set of inputs: both if the gpu had initialized already, and if it had not.

Properly validating software is possible, but it takes a lot more work than just seeing if Windows boots up on it.