Is it possible to put a cover over the sensor to block out the light?

 

Of course. Black electrical tape or silicone any small black cap that fits over it. Headlights will remain on unless switch is turned to off position.

 

You can also pop the sensor out the dash and bypass the sensor with a wire and you just have a on and off switch with the auto switch.

 

That's the correct way to test photocell sensors in bright day light!
You could also test by driving through a road tunnel.

 

That won't work...Unless the Tunnel is long. The GM Ganja Tech-ies built a delay in the system in case you go thru a short tunnel. Best way to test is with a dark cover and a stopwatch. Time it. Or have your wife or Girlfriend smear some eyelash goop on there while your doing 99 MPH.