test: refector parallel/test-http.js

* favor ’===’ over in ’==’
* favor ’assert.strictEqual’ over ’assert.equal’
* favor ’const’ over ’var’

PR-URL: https://github.com/nodejs/node/pull/8471


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