mathlib3

bbd198e1 - chore(test/gmonoid): Add a test of the gmonoid API (#18197)

chore(test/gmonoid): Add a test of the gmonoid API (#18197) This is taken from my CICM 2022 paper. I don't think the instance is canonical enough to be in mathlib, but it's useful to have a test that verifies it can be constructed.