Providing a passcode is testimony because by it's nature it proves you know the password and have access to the device. The government must prove that you have access to that device. That can also get you into a catch-22 because what if you don't know the password, and what if the device isn't yours? Touching your finger to a sensor is something they can do to any suspect, guilty or innocent. The fact that your finger is the one that unlocks the phone does prove that you have access to it, but that is not forcing you to incriminate yourself anymore than giving a DNA sample or your fingerprints "incriminate" you if they match the evidence the police already have. You might as well argue against letting eye-witnesses see you in a lineup because that would incriminate you.
It seems that many people don't understand the purpose of the right against self incrimination. Hundreds of years ago it was common to force people to confess to crimes even using torture and that was the primary means of getting a conviction. The police would (hopefully) figure out who they think probably committed a crime and just browbeat or torture them until they confessed. You shouldn't have to answer any questions that relate to knowledge that may provide evidence against you. There is no protection against harmlessly making you do something to allow evidence to be interpreted, such as standing in a line-up, getting your fingerprints, getting a dna sample, getting dental impressions to match to bite marks, getting hair samples, getting your blood for blood type testing, etc. That would include placing your finger on a sensor.