test: fix test-sync-io-option

Wait for all the data to be available in stderr before checking its
contents. In FreeBSD this was failing because stderr data was being
emitted in multiple chunks. 4 WARNINGS are printed instead of 2 for
each sync call inside readFileSync. require('fs') does not print any
trace.

PR-URL: https://github.com/nodejs/io.js/pull/1734


Reviewed-By: Trevor Norris <trev.norris@gmail.com>