test: improve test-http-chunked-304

* change the nested functions call to run tests in parallel
* use const and let instead of var
* use common.mustCall to control functions execution
* use assert.strictEqual instead of assert.equal
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10462


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>