Now in Cboltd's defense - The ABC store is directly across from the Taco Bell. The shots could have come from the car in the Taco Bell Parking lot, but it would have taken too long for Malvo to shoot, get out, run across the street and be rifling the purse. The cop got there in under a minute.
So yes it could have been a test run of the car setup, and Malvo may have been acroos the street to see what he could see and hear. Then he could also rob the victims afterward. The accuracy of the shots (Neck) also points to Muhammed taking the shots if this is the case.
The cop see's malvo chases him, Muhammed leaves the Taco Bell and nonchalantly heads to the Interstate. Malvo Hits the Drainage ditch, goes under the road under the fence and down the hill to where Muhammed is waiting for him on the side of I-85 - into the car and gone.