And just for the purpose of turning the engine over, you don't even need the circuit to the key. You can hook up the battery-to-solenoid, solenoid-to-starter and battery to ground (both the engine AND the body) and simply jumper power from the battery side of the solenoid to the small post closest to it. VV (voila and vrooooom!).
Just be sure to MAKE SURE THAT THE TRANSMISSION IS IN NEUTRAL!!! and that the negative battery terminal is just barely snug. That way you can yank the cable off if the starter sticks in the on position.
When testing it's just always good to be cautious.
Paul