test: fix race in parallel/test-vm-debug-context

Fix a race condition in parallel/test-vm-debug-context where the 'exit'
event for the child process is emitted before the first and only 'data'
event for the child process's stderr stream.

I considered deferring the 'exit' event in lib/child_process.js until
all stdio streams have been closed but I realized that's not going to
work when the child process spins off grandchildren that keep the stdio
file descriptors alive.

Fixes: https://github.com/iojs/io.js/issues/1291
PR-URL: https://github.com/iojs/io.js/pull/1294


Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>