How is it testing the sensor?

Everything I've ever seen and heard backs up what Dan said. The test button on detectors (smoke and CO) simply tests that the unit has power, and the alarm speaker functions. Actual alarm testing requires real smoke or CO to be present.

The behavior you see when there is a delay is intentional, caused by a timed relay. This is to allow time to press the button and move away, to avoid being right next to the alarm when it sounds. Not all alarms have this feature though.