test: add mustCall to test-dgram-connect-send-multi-buffer-copy.js

PR-URL: https://github.com/nodejs/node/pull/27465


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Ouyang Yadong <oyydoibh@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>