Doesn't work for all cartridges (I didn't run Opus that long, but it could keep going). Works with Super Mario.

But to answer your question, it's in the readme: https://github.com/dbreunig/nesformer#how-it-was-checked

Do you test every possible input/output combination on any CPU?

No, because that would be impossible.

You perform a series of more specific tests. In this case, it's "working" in so far as that Super Mario Bros. appears to be playable, for example.

Does not mean that it behaves perfectly according to some spec, or identical to another CPU (like the NES's 6502), of course, but it's clearly a working CPU to some 6502-compatible extent.