engine power

That test is designed to show what the brakes are doing with the engine running (to provide vacuum to the vacuum booster) when you slam on the brakes, as though there were an emergency. You're not holding the brakes and trying to turn the tires, you're slamming on the brakes, letting off of them, and seeing if the calipers stay locked for the 30 seconds you mentioned. Think replication of the problem in a controlled environment. There should not be small imperfections on the brakes, the brakes release because there is a spring in the vacuum booster that overcomes the booster when the brakes are not applied. That coupled with the fact that the equal and opposite reaction of pressing brake pads into a brake rotor is that they want to push away from the brake rotor. When you let off the brakes, they do this proportionally to the amount of decreasing force applied to the calipers. You may think next, "wouldn't the pads always be touching the rotors?", and the answer is yes. Always.

Ok, but you only need one person. If you try to turn the wheels by hand while holding the brake, they will not turn, so that wouldn't lead to a conclusion. Also, you need to slam on the brakes just like you would in an emergency situation as described. That is exclusively to replicate what you described as emergency braking (hard braking). This will not test anything else. Master cylinder failure and vacuum booster failure occur in more than one way, and suffice to say, no, it would not mean they are hard to stop. In fact, I suspect they may be the source of the problem. The accelerator has nothing to do with the brake system. The brakes release because the potential energy stored from the compression of the brakes on the rotor is the product equal and opposite reaction. In other words, squeezing the brakes under high pressure means the brakes want to release with the same amount of force. If you have imperfections on your brakes, you will have brake noise. They should be smooth.