[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 5cec8b85e5)
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>

src/net/http/httptest/server.go

@@ -316,6 +316,13 @@ func (s *Server) wrap() {
 	s.Config.ConnState = func(c net.Conn, cs http.ConnState) {
 		s.mu.Lock()
 		defer s.mu.Unlock()
+
+		// Keep Close from returning until the user's ConnState hook
+		// (if any) finishes. Without this, the call to forgetConn
+		// below might send the count to 0 before we run the hook.
+		s.wg.Add(1)
+		defer s.wg.Done()
+
 		switch cs {
 		case http.StateNew:
 			s.wg.Add(1)