net/http: fix benchmark goroutine leak

New detection because of net/http now using TestMain.

Fixes #9033

LGTM=iant
R=golang-codereviews, iant
CC=adg, golang-codereviews, rsc
https://golang.org/cl/170210043

src/net/http/serve_test.go

@@ -2819,6 +2819,7 @@ func benchmarkClientServerParallel(b *testing.B, parallelism int, useTLS bool) {
 				InsecureSkipVerify: true,
 			},
 		}
+		defer noVerifyTransport.CloseIdleConnections()
 		client := &Client{Transport: noVerifyTransport}
 		for pb.Next() {
 			res, err := client.Get(ts.URL)