http2: discard logger output in invalid response tests

In invalid response tests logger write error messages to stderr and spam
test output.

Since we know response are invalid in these tests we can safely discard
logger output.

Fixes golang/go#22850

Change-Id: Id8c97be910f0cf7dbe2380ba632960364bc8478b
Reviewed-on: https://go-review.googlesource.com/80235
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/transport_test.go

@@ -13,6 +13,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"log"
 	"math/rand"
 	"net"
 	"net/http"
@@ -2291,6 +2292,11 @@ func TestTransportReadHeadResponse(t *testing.T) {
 }
 
 func TestTransportReadHeadResponseWithBody(t *testing.T) {
+	// This test use not valid response format.
+	// Discarding logger output to not spam tests output.
+	log.SetOutput(ioutil.Discard)
+	defer log.SetOutput(os.Stderr)
+
 	response := "redirecting to /elsewhere"
 	ct := newClientTester(t)
 	clientDone := make(chan struct{})
@@ -3383,6 +3389,11 @@ func TestTransportRetryHasLimit(t *testing.T) {
 }
 
 func TestTransportResponseDataBeforeHeaders(t *testing.T) {
+	// This test use not valid response format.
+	// Discarding logger output to not spam tests output.
+	log.SetOutput(ioutil.Discard)
+	defer log.SetOutput(os.Stderr)
+
 	ct := newClientTester(t)
 	ct.client = func() error {
 		defer ct.cc.(*net.TCPConn).CloseWrite()