test: fix errors in test-buffer-alloc.js

This fixes test code that was mistaken landed by me this morning. (Code
& Learn rush gone bad.) Sorry. Please fast track.

PR-URL: https://github.com/nodejs/node/pull/23645


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>