test: fix and improve error message

Add unexpected value to dgram socket type assertion and fix the size
entry.

PR-URL: https://github.com/nodejs/node/pull/18449


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>