test: fix test-child-process-stdout-flush-exit

Make sure all the stdout data is available before
performing the assertions.

Fixes: https://github.com/nodejs/io.js/issues/944
PR-URL: https://github.com/nodejs/io.js/pull/1868


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>