test: fix flakiness of stringbytes-external

The tests used to rely on precise timing of when a JavaScript object
would be garbage collected to ensure that there is enough memory
available on the system. Switch the test to use a malloc/free pair
instead.

Ref: https://github.com/nodejs/node/pull/5945
PR-URL: https://github.com/nodejs/node/pull/6039


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: evanlucas - Evan Lucas <evanlucas@me.com>
Reviewed-By: Trott - Rich Trott <rtrott@gmail.com>