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.

Cherry picked from https://github.com/jbergstroem/io.js/commit/31940738e2d256e91c1c694c5219b8f840c629ff
Original commit message follows:
  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>

Conflicts:
	tools/test.py

Conflicts:
	tools/test.py

PR-URL: https://github.com/joyent/node/pull/25686


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>