You're over thinking this one - Simulate darkness by covering the light sensor (not the PIR sensor) with a piece of insulating tape.
Unfortunately your pictures don't show either sensor, so we can't tell you which exactly component to cover up, but a PIR sensor usually sits behind a segmented lens, has a metal body with a square or rectangular glass "window". A light sensor is usually in a plastic package.
Avoid getting finger prints on the PIR sensor window - it will reduce its sensitivity.