test: eliminate multicast test FreeBSD flakiness

test-dgram-multicast-multi-process was flaky on FreeBSD and Raspeberry
Pi. This refactoring fixes the issue by eliminating a race condition.

Fixes: https://github.com/nodejs/node/issues/2474
PR-URL: https://github.com/nodejs/node/pull/4042


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>