[iamed2]

Invenia's approach to interface testing ("Development with Interface Packages" on our blog) does some of the things you suggest as a standard of practice, by providing tools to check correctness that implementers can use as part of package tests. ChainRulesTestUtils.jl is a decent example (although this one doesn't come with fake test types). I think this is typically good enough, and only struggles with interface implementations with significant side effects.

One little win could be publishing interface tests like these for Base interfaces in the Test stdlib. I appreciate that the Generic* types are already exposed in the Test stdlib!