test: option to run a subset of tests

With this option, tests can be divided in groups and only one is run
from each group. This enables us to parallelize the tests across
multiple machines.

PR-URL: https://github.com/nodejs/io.js/pull/2260


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Alexis Campailla <alexis@janeasystems.com>