test: improve assertion message in internet dgram test

When an error occurs unexpectedly, let the user know what the error is.

PR-URL: https://github.com/nodejs/node/pull/30545


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>