What this game actually checks
Every button on the drawing lights up once, in random order. If you can press each one before the timer runs out, all sixteen work. A button that never responds costs you a life, and the game tells you afterwards which ones you missed. That is the same information the plain tester gives you, just faster to get through.
The two triggers count as pressed once you pull them past halfway, so a trigger with a worn sensor may still pass here. If you want the exact pressure reading, use the main tester instead.
Why a game and not a checklist
Pressing sixteen buttons one by one is dull, and it is easy to skip one by accident and think you tested everything. A timer and three lives keeps you honest: you cannot move on until the button actually registers.
If the game will not start
The play button stays greyed out until your browser sees a controller. Plug it in, then press any button on the pad. Browsers hide gamepads until you press something, which is a privacy rule rather than a fault. Chrome and Edge are the safest choice.