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: #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>

test/benchmark/test-benchmark-buffer.js

@@ -13,7 +13,7 @@ runBenchmark('buffers',
                'charsPerLine=6',
                'encoding=utf8',
                'endian=BE',
-               'len=2',
+               'len=256',
                'linesCount=1',
                'method=',
                'n=1',