test: fix flaky test-child-process-fork-dgram

`test-child-process-fork-dgram` is unreliable on some platforms,
especially FreeBSD and AIX within the project's continuous integration
testing. It has also been observed to be flaky on macos.

* Confirm child has received the server before sending packets
* Close the server instance on the parent or child after receiving a

Refs: https://github.com/nodejs/node/pull/8697
Fixes: https://github.com/nodejs/node/issues/8949
Fixes: https://github.com/nodejs/node/issues/8271
PR-URL: https://github.com/nodejs/node/pull/9098


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>