test: refactor test-dgram-setBroadcast.js

This test wasn't actually working, as sockets were being closed,
allowing the test to exit before any assertions were actually
run. This commit refactors the test to maintain the same intended
semantics.

PR-URL: https://github.com/nodejs/node/pull/11252


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