net/http: add onClose hook to fake net listener

net/http: add onClose hook to fake net listener

Avoids a race condition: If we set an onClose hook on a conn
created by a listener, then setting the hook can race with
the connection closing.

Change-Id: Ibadead3abbe4335d41f1e2cf84f4696fe98166b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/658655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Damien Neil <dneil@google.com>