Watts are watts and if you are on the same device(s) for testing and training you shouldn’t fuss over this.
However - as Chad pointed out - different strokes for different folks. I wouldn’t recommend this for a 20min test, but on another ride/workout or an 8 min test EXPERIMENT. High cadence is better for some people (saving their muscles) where some get better turning a bigger gear at a lower cadence (saving their lungs and HR).
So while watts are watts, you might personally be able to accomplish more watts using one method or another.
You might find that a massive resistance, little gear, and more muscular effort works for you. You might find that little resistance, pushing a big gear and a high cadence works better for you.