test: refactor code in test-cluster-http-pipe

* use common.mustCall to control the functions execution automatically
* use const instead of var
* use assert.strictEqual instead assert.equal
* use assert.ifError instead of throw error

PR-URL: https://github.com/nodejs/node/pull/10297


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>