test: refactor the code in test-http-keep-alive

* use common.mustCall to control the functions execution automatically
* use let and const instead of var
* use assert.strictEqual instead assert.equal

PR-URL: https://github.com/nodejs/node/pull/10350


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>