test: add test for fork() + shell

This commit verifies that the child_process fork() method does
not honor the shell option.

Refs: nodejs/node#15299
PR-URL: nodejs/node#15352
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

Author: cjihrig