test: fix test case in test-child-process-fork-dgram.js

PR-URL: https://github.com/nodejs/node/pull/24459


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>