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.

Author: Trott

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',