test: test cluster worker disconnection on error

This test checks that ownerless cluster worker handles are closed
correctly on disconnection.

Fixes: https://github.com/nodejs/node/issues/6561
PR-URL: https://github.com/nodejs/node/pull/6909


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>