test: use correct size in test-stream-buffer-list

The `n` argument of `BufferList.prototype.concat()` is not the number
of `Buffer` instances in the list, but their total length when
concatenated.

Backport-PR-URL: https://github.com/nodejs/node/pull/19483
PR-URL: https://github.com/nodejs/node/pull/18239


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>