Edit: Seems you deleted your post…but below is my answer anyway.
It depends on what you’re looking to test. Generally, any test that you want to be representative of a situation for a certain device, needs to approximate said situation.
If you’re just testing whether or not a specific ESC can sustain X amount of amps, then generally you would use FOC open loop into a motor that can handle that current, and then monitor thermals.
Since that generally still leaves the battery in current relatively low, you could then set up a dynamometer to simlate torque load on a motor at a certain RPM, and funnel that back to require more current draw from the battery.
A power supply likely isn’t going to be able to provide 50-75v at tens of amps, unless you’ve got quite the budget. So a battery of a representative size would be appropriate.
Generally, most folks who are interested in testing such things simply use the most practical test jig available, which is an actual eskate they’ve built, and then substitute the part in to test.
Of course, that’s dangerous should the part fail, but it’s usually cheaper for someone who doesn’t have a whole shop’s worth of stuff to make testing jigs.
You could also design a specific PCB based testing jigs like what Future Motion does, but that takes engineering talent and money.