test: modernize syntax, add strict checks

Changed `var` to `const`, strings to template literals, and
assert.equal to assert.strictEqual where appropriate.

PR-URL: https://github.com/nodejs/node/pull/8841


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>