test: correct spelling of 'childProcess'

Per: https://github.com/joyent/node/pull/8715

Cleans up a minor spelling error in two tests.

PR-URL: https://github.com/nodejs/node/pull/2389


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>