test: complete coverage of lib/child_process.js

This commit adds a test which brings coverage of
lib/child_process.js to 100%. It adds coverage for the call to
uv.errname() in execFile()'s exithandler() function.

PR-URL: https://github.com/nodejs/node/pull/12367


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>