test: refactor test-http-response-splitting

* move repeated code to function
* remove unneeded `common.mustCall()` usage with function arguments that
  are not callbacks
* add error message checking

PR-URL: https://github.com/nodejs/node/pull/11429


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>