test: add "mustCall" for test-net-buffersize

PR-URL: https://github.com/nodejs/node/pull/27451


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ouyang Yadong <oyydoibh@gmail.com>