test: changed var to const, added strict equal checks

Changed var to const where appropriate. Substituted
assert.strictEqual for assert.equal for better type checks.

PR-URL: https://github.com/nodejs/node/pull/8762


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>