test: add coverage for spawnSync() killSignal

This commit adds a test for the killSignal option to spawnSync(),
and the other sync child process functions by extension.

PR-URL: https://github.com/nodejs/node/pull/8960


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>