Attempt to build DIY high current power supply from car alternator?
Alternators are designed to generate enough voltage to prevent the battery from discharging at the engine idle speed. Speed and field current determine the voltage. At idle, the regulator lets the maximum field current flow to the alternator. As engine speed increases, the regulator cuts back the current to prevent excess voltage. For your purposes, you would only need to drive the alternator at the speed it would run at the normal idle speed of the engine. You can operate the alternator without a battery, but you do need the proper voltage regulator and a 12-volt DC power supply for the field. If you operate the alternator at a voltage that is too high, the initial starter current will be higher and more difficult to provide.

Once the starter is at full speed, the current should drop somewhat, but probably not as much as might be expected with no load, because the field is connected in series with the armature. I would think that a proper bench would include measuring the idle current. There must be some guidelines about that someplace.

One thing that might help with the testing power supply would be to put a small value of resistance, perhaps 1/20 ohm, in series with the starter before connecting it to the power supply and then short the resistor when the motor has picked up some speed. You might be able to make the resistor from a piece of stainless steel.