test: fix flaky SmartOS test

SmartOS has an issue where it will trigger ECONNREFUSED when it should
not. See https://smartos.org/bugview/OS-2767.

This change adds logic to test-net-server-max-connections.js to work
around the issue.

Fixes: https://github.com/nodejs/node/issues/2663
PR-URL: https://github.com/nodejs/node/pull/3830


Reviewed-By: James M Snell <jasnell@gmail.com>