test: deflake test-http2-large-write-multiple-requests
If the server is not referenced, it might go away too soon and the client may not get enough ends for it to close itself, resulting a timeout. This patch updates the test to simply close the server when enough requests have been processed, and keep the server referenced while the test is ongoing.
Drive-by: add more logs to facilitate debugging. Refs: https://github.com/nodejs/reliability/issues/791