Why aren't airliners spin-tested?
1. Spins are less frequent on large aircraft

They are less maneuverable
They are flown by more experienced pilots
They have more protection systems (e.g. flight envelope protection, yaw damper) to avoid uncontrolled flight

2. Spins may cause structural damage on large aircraft
The larger the aircraft, the lower its tolerance to stress. While one can quite comfortably spin a Cessna or Cherokee, attempt one on a Boeing 777 and things will break. Gear doors, the bolts holding the vertical stabilizer as well as cabin accessories are not designed to handle stress in that direction, not to mention engine flame-out.
3. Spins may not be recoverable
As discussed in this question.