test: fix test-sync-io-option

This test was failing occasionally both locally and on CI. Switched
from using spawn to execFile for a more reliable test.

Fixes: https://github.com/nodejs/io.js/issues/1837
PR-URL: https://github.com/nodejs/io.js/pull/1840


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