I'm still testing positive and the illness manifests itself in everything, including ability to take photographs or even think straight. Ruth is 3 days ahead of me and after nearly 2 weeks she isn't yet in the clear.
A silicone wafer is an excellent test subject, but do note there are aspects of 'macro' work it won't help with. You need to resort to stacking for example to find whether bad corners are due to the lens having insufficient coverage or there is excessive curvature of field. Also, unless you go well beyond 4X or so, many details are too small to be adequately resolved even by the best of lenses. The wafer will help in showing signs of setup instability or camera shutter-induced movement, though. Chromatic aberrations should be obvious, but do note the light source can add aberrations on its own as the structures on the wafer can act as micro prisms. So a narrowband source is preferable if it can be had.
Do remember to keep the silcone wafer stored with cardboard or similar to stiffen it and wrap it into plastic to keep dust away plus avoid scratching its surface, which is pretty delicate. I bought a wafer many years ago and it cost me next to nothing back then. No idea what the situation currently is.
