test: improve test-https-agent.js

On line 40: replace '==' with '==='

On line 52: replace 'assert.equal' with 'assert.strictEqual'

Added some comments.

Changed 'var' to 'const' where possible.

Replaced console.log(res.statusCode); with and assertion.

Rather than logging the https request status on every loop it will now
assert the https status is correct on every loop.

Changed the error listener to throw the error rather than log it.

PR-URL: https://github.com/nodejs/node/pull/8517


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