How could a light source like the sun be approximated on Earth for testing, given how much brighter it is in space than anything else?
You can match the lux count of the sun, in a small area. The amount of solar power that shines on a satellite is about 1300 W/m2 in Earth orbit.
This is done at e.g. the ESA Large Space Simulator at ESTEC, in the Netherlands.
This provides a horizontal solar beam of 6-m diameter with excellent uniformity and very high long- and short-term stability (less than 0.5%). An intensity level of one solar constant (the standard solar energy received at Earth distance from the Sun, equivalent to 1380 watts per square metres) can be produced by operating 12 of 19 xenon lamp modules at a nominal power of 20 kilowatts per lamp. With all lamps at full power, engineers can achieve flux in excess of 2700 watts per square metre.