[release-branch.go1.16] net/http/httptest: wait for user ConnState hooks

Ensure that user ConnState callbacks have completed before returning
from (*httptest.Server).Close.

Fixes: #49851
Updates: #37510
Updates: #37505
Updates: #45237

Change-Id: I8fe7baa089fbe4f3836bf6ae9767c7b1270d1331
Reviewed-on: https://go-review.googlesource.com/c/go/+/304829


Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 5cec8b85)
Reviewed-on: https://go-review.googlesource.com/c/go/+/367516


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>