I have a luxury: for a past customer, I designed and built a dual channel, 0-18V, +/-60A controlled source/load. So, of course I keep one on hand so I can test anything they can.
Test a 12V supply? No problem, plug it in and crank up the amps! Load voltage and current are displayed on meters.
If you aren't so fortunate, any random load resistance is helpful. If you need a lot cheap, try looking inside industrial hulks at the scrapyard -- there may be power resistors inside! However you do it, you'll want a collection of assorted values, so you can mix and match for whatever loads you like. Tip: use a
resistor network calculator to find the optimum network for a given maximum load.
Tim