My car was at a shop being repaired and as the mechanic drove the car to test it, it was hit by another car. Who is at fault?

Typically, under the law of bailments, when you pay somebody to repair your car, it constitutes a "bailment for hire", and the person who takes charge of your car is liable if they fail to exercise ordinary care to preserve the condition of your car.  Thus, typically, a service shop would be liable under the circumstances you describe for the negligence of its employee, as that would reflect a failure to exercise due care.

Also, if the vehicle is returned in damaged condition, there is a prima facie case for liability on the part of the bailee (the repair shop) and it then the bailee's duty to prove that they did not violate their duty to exercise ordinary care. A local attorney should be able to advise you on the specifics of auto accident insurance law in your state.