test: cleanup test-child-process-disconnect.js

Changed var --> const and let
Changed assert.equal --> assert.strictEqual

PR-URL: https://github.com/nodejs/node/pull/8602


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