os/exec: quiet distracting log output during test

TLS handshake failures didn't use to log, but do in Go 1.3.
Shut it up so the actual failure can be seen in e.g.
http://build.golang.org/log/ede7e12362a941d93bf1fe21db9208a3e298029e

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/87870043

src/pkg/os/exec/exec_test.go

@@ -13,6 +13,7 @@ import (
 	"fmt"
 	"io"
 	"io/ioutil"
+	"log"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -401,11 +402,15 @@ func TestExtraFiles(t *testing.T) {
 
 	// Force TLS root certs to be loaded (which might involve
 	// cgo), to make sure none of that potential C code leaks fds.
-	ts := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		w.Write([]byte("Hello"))
-	}))
+	ts := httptest.NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {}))
+	// quiet expected TLS handshake error "remote error: bad certificate"
+	ts.Config.ErrorLog = log.New(ioutil.Discard, "", 0)
+	ts.StartTLS()
 	defer ts.Close()
-	http.Get(ts.URL) // ignore result; just calling to force root cert loading
+	_, err = http.Get(ts.URL)
+	if err == nil {
+		t.Errorf("success trying to fetch %s; want an error", ts.URL)
+	}
 
 	tf, err := ioutil.TempFile("", "")
 	if err != nil {