test: fix test-benchmark-buffer

Using `len=2` in test-benchmark-buffer was resulting in a `RangeError`
in buffer-base64-encode.js. Change to `len=256` which works in all
buffer benchmarks.

PR-URL: https://github.com/nodejs/node/pull/27260


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>