test: modernize js and tighten equality checking

Changed var --> const and let.
Changed assert.notEqual --> assert.notStrictEqual
Fixed comment spelling

PR-URL: https://github.com/nodejs/node/pull/8618


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>