test: add coverage for execFileSync() errors

This commit adds coverage for errors returned by execFileSync()
when the child process exits with a non-zero code.

PR-URL: https://github.com/nodejs/node/pull/9211


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>