test: update test/parallel/test-child-process-send-utf8.js

change var to const
use arrow function in callback
use strict equal

PR-URL: https://github.com/nodejs/node/pull/8594


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>