test: support writing test output to file

This is a minimal effort to support test output written both to
stdout and file in order to get our buildbots understanding
test output.

PR-URL: https://github.com/iojs/io.js/pull/934


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>