test: refactor test-child-process-ipc

Change var to const or let.
Change assert.equal() to assert.strictEqual().

PR-URL: https://github.com/nodejs/node/pull/9990


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>