test: inherit JOBS from environment

In some virtualized environments the amount of available resources
are misleading; for instance `multiprocessing.cpu_count()` on our current
4-core smartos vm's returns `48`. This is not a bug, merely how the vm
host provides information about available hardware.

Avoid running into issues by overriding `cpu_count()` with `JOBS`.

PR-URL: https://github.com/nodejs/node/pull/4495


Reviewed-By: Brian White <mscdex@mscdex.net>