test: reland changes from 11c1bae7

Reland the changes from commit 11c1bae7 ("lib: make --debug-port work
with cluster") that were temporarily backed out to cherry-pick commits
from joyent/node.

PR-URL: https://github.com/iojs/io.js/pull/501


Reviewed-By: Bert Belder <bertbelder@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>