* Make partial= test helper take exactly two arguments

Fixes #21454.

`(= 2) ;=> true` and `partial=` followed that pattern. It leads to
confusing test assertions that silently succeed.

It's possible to extend this to support >= 2 arguments instead, but I'm
keeping it simple for now.

Co-authored-by: Cam Saul <1455846+camsaul@users.noreply.github.com>