test: fix flaky test-cluster-shared-leak

Test was flaky on centos7-64 due to an uncaught ECONNRESET
on the worker code. This catches the error so the process
will exit with code 0.

Fixes: https://github.com/nodejs/node/issues/5604
PR-URL: https://github.com/nodejs/node/pull/5802


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