test: modernize JS and tighten equality checking

Changed var --> const and let.
Changed assert.equal !== --> assert.notStrictEqual
Correctly aligned argument

PR-URL: https://github.com/nodejs/node/pull/8580


Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>