test: add child_process.exec() timeout coverage

This commit adds coverage for the timeout option used by
child_process exec() and execFile().

PR-URL: https://github.com/nodejs/node/pull/9208


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>