test: speed up stringbytes-external test

test-stringbytes-external tends to take quite a while on slower
hardware. A lot of the time is taken by creating a new buffer that is
very large. The improvements come from reusing the same buffer.

PR-URL: https://github.com/nodejs/node/pull/3005


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>