What is the simplest method to simulate a solar panel on a current/ voltage regulated PSU?
(1) Required complexity depends on required fit to a "real" panel, but a moderately good result can be had from
 a constant current source
 feeding a voltage  regulator,
 with a series resistor in the output
 plus a high resistance resistor bypassing the voltage regulator to provide a soft voltage knee. 

This would require an eg LM317 + 1 resistor to provide constant current,
 followed by an eg LM317 2 resistors as the voltage regulator
 plus a series resistor to provide some "droop" as increasing current is drawn
 plus a resistor to output from Voc or a resistor divider to make Voc from a higher voltage.
 throw in a few filter capacitirs
 and start playing with the series resistor and output knee softener to get the shape you want.

If using an LM317 and wanting 0.5 or so volts out - ie 1 PV Si cell - a few diodes in series with the output will drop enough voltage to offset the otherwise 1.25V minimum.

(2) At low power levels - and you can test MPPT algorithms at low power and scale them directly - power level makes little difference -  a small photovoltaic cell and a variable brightness LED light of a few Watts output - or a steady few Watts of light that can be moved, makes a very simple, relatively low cost near perfect simulation of itself.

Spectral output of light is quite different from standard AM1.5 1 kW/m^2 sunlight but this is reasonably irrelevant here.