Active load using MOSFET
Depending on what aspect(s) of performance you're looking to evaluate, I would suggest using  low inductance fixed power resistors of various values to evaluate the step response. One resistor for the static load (if any) and a second for the dynamic load. 

PWM is going to complicate things to the extent that you may not be able to depend on the results. 

I like to use commercial 'electronic loads' that can simulate an constant (programmable) resistance, current or constant power load, and that can programmed to cycle the load so you can look at the response on an oscilloscope, but of course that's a bit more complex. 

If constant current is all you need it would be pretty simple to use a DAC and a voltage-to-current converter, and you could program the static and dynamic loads digitally on your micro. If your DC-DC converter is supposed to be constant output voltage (as opposed to, say, a constant current type for, say, LED driving) then constant current should be pretty close to the effect of constant resistance if the output voltage doesn't change much.