test: refactor test-http-pause-resume-one-end

* setTimeout() with no duration -> setImmediate()
* var -> const
* remove unused variable `chunk`

PR-URL: https://github.com/nodejs/node/pull/10210


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>