test: simplify for loop in test-buffer-zero-fill-cli.js

PR-URL: https://github.com/nodejs/node/pull/26799


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>