net/http: prevent infinite wait during TestMissingStatusNoPanic

If the client request never makes it to the server, the outstanding
accept is never broken. Change the test to always close the listening
socket when the client request completes.

Updates #45358

Change-Id: I744a91dfa11704e7e528163d7669c394e90456dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/319275


Trust: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>