benchmark: enable assert benchmark with short len

`deepequal-typedarrays.js` throws if `len` is set to 100 or less due to
a hardcoded index. Calculate the index based on `len` so benchmark can
be run with small `len` values if desired.

PR-URL: https://github.com/nodejs/node/pull/15174


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>