“There are so many aspects of the implementation that can go wrong: the sampling distribution, the proper weighting of the samples, a mistake in the PDF, a pi or a 2 factor forgotten somewhere… Those errors tend to be subtle and can result in a render that still looks reasonable. Nothing looks more like a correct shading than a slightly incorrect one.”
A 100% reflective object under uniform light should vanish into the background. If it shows up, the renderer is adding or losing energy somewhere. The correct output is an empty image, and that is the point. Eyeballing a render tells you it looks fine. The test tells you whether the math is lying.