crypto/tls: fix printing of OpenSSL output when updating a test fails.

When updating the test data against OpenSSL, the handshake can fail and
the stdout/stderr output of OpenSSL is very useful in finding out why.

However, printing that output has been broken for some time because its
no longer sent to a byte.Buffer. This change fixes that.

Change-Id: I6f846c7dc80f1ccee9fa1be36f0b579b3754e05f
Reviewed-on: https://go-review.googlesource.com/30823
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

handshake_client_test.go

@@ -412,7 +412,7 @@ func (test *clientTest) run(t *testing.T, write bool) {
 		childProcess.Process.Kill()
 		childProcess.Wait()
 		if len(recordingConn.flows) < 3 {
-			childProcess.Stdout.(*bytes.Buffer).WriteTo(os.Stdout)
+			os.Stdout.Write(childProcess.Stdout.(*opensslOutputSink).all)
 			t.Fatalf("Client connection didn't work")
 		}
 		recordingConn.WriteTo(out)