test: improve child_process tests

Replaced var keyword with const and let in the tests for child process
stdin and stdio.

PR-URL: https://github.com/nodejs/node/pull/8617


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>