build: allow test-ci to run tests in parallel

Run tests in parallel if the environment variable JOBS
(which should contain a number of parallel jobs) is set.

PR-URL: https://github.com/nodejs/node/pull/6208


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>