test: clean up http-set-trailers

* remove shared state of request counting from each listener by using
  callbacks to report test finish. This also fixes slight race condition
  where one of the request could finish before the other was taken into
  account resulting in ECONNREFUSED due to premature server.close()
* slightly move code for better cohesion
* fix error comment in testHttp10 'Trailer ...' -> 'No trailer ...'

PR-URL: https://github.com/nodejs/node/pull/30522


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>