test: handling failure cases properly

Refer: https://github.com/nodejs/io.js/issues/1543

When this test fails, it leaves dead processes in the system. This
patch makes sure that the child processes exit first, in case of
errors.

PR-URL: https://github.com/nodejs/node/pull/2206


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>