net/http: fix flaky test

Prevent idle transport on race condition.

Fixes #7847

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/96230044

src/pkg/net/http/transport_test.go

@@ -1553,8 +1553,10 @@ func TestTransportSocketLateBinding(t *testing.T) {
 	dialGate := make(chan bool, 1)
 	tr := &Transport{
 		Dial: func(n, addr string) (net.Conn, error) {
-			<-dialGate
-			return net.Dial(n, addr)
+			if <-dialGate {
+				return net.Dial(n, addr)
+			}
+			return nil, errors.New("manually closed")
 		},
 		DisableKeepAlives: false,
 	}
@@ -1589,7 +1591,7 @@ func TestTransportSocketLateBinding(t *testing.T) {
 		t.Fatalf("/foo came from conn %q; /bar came from %q instead", fooAddr, barAddr)
 	}
 	barRes.Body.Close()
-	dialGate <- true
+	dialGate <- false
 }
 
 // Issue 2184