test: refactor test-http-client-readable

* var -> const
* remove function names where V8 inference is as good or better
* add function names where there is no V8 inference
* assert.equal -> strictEqual
* move assertion from exit handler to response end handler

PR-URL: https://github.com/nodejs/node/pull/9344


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>