test: validate 'expected' argument to mustCall()

instead of silently overwriting invalid values with the default

PR-URL: https://github.com/nodejs/node/pull/10692


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>