test: ensure _handle property existence

`test-stdtout-close-unref.js` will fail if `process.stdin._handle` does
not exist. On UNIX-like operating systems, you can see this failure this
way:

    ./node test/parallel/test-stdout-close-unref.js < /dev/null

This issue has been experienced by @bengl and @drewfish in a Docker
container. I'm not sure why they are experiencing it in their
environment, but since it is possible that the `_handle` property does
not exist, let's use `child_process.spawn()` to make sure it exists.

PR-URL: https://github.com/nodejs/node/pull/5916


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>