Gonna put me on the spot, eh? Well, first you would need a profile of the injector, time to fully open, time to fully close, volume, etc. I'm not an expert here, but I do know that injectors work at alot higher frequency. What I'm saying is, you need to design your tester to the injector specs.Ok then what would you suggest..
Again, not an expert, but I think injectors would need to be driven hard and fast. And so, pay particular attention to the gate drive of your pass device. You'll need a minimum drive, based on gate capacitance and minimum turn on time. Someone earlier mentioned a driver, maybe it's worth taking a look at. Bottom line, after you have all the requirements for testing the specifications for your injector, then we can begin to design a circuit for it.