test: update test/parallel/test-child-process-stdio.js

Refs: nodejs/code-and-learn#56(comment)

Replace the equal assetion with strictEqual.
Replace the notEqual assertion with strictNotEqual.

PR-URL: https://github.com/nodejs/node/pull/8584


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>