refactor header writing to append to a byte slice (#3646)

This avoids having to allocate a bytes.Buffer.

internal/testutils/testutils.go

@@ -1,7 +1,7 @@
 package testutils
 
 import (
-	"bytes"
+	"fmt"
 
 	"github.com/lucas-clemente/quic-go/internal/handshake"
 	"github.com/lucas-clemente/quic-go/internal/protocol"
@@ -13,9 +13,11 @@ import (
 
 // writePacket returns a new raw packet with the specified header and payload
 func writePacket(hdr *wire.ExtendedHeader, data []byte) []byte {
-	buf := &bytes.Buffer{}
-	hdr.Write(buf, hdr.Version)
-	return append(buf.Bytes(), data...)
+	b, err := hdr.Append(nil, hdr.Version)
+	if err != nil {
+		panic(fmt.Sprintf("failed to write header: %s", err))
+	}
+	return append(b, data...)
 }
 
 // packRawPayload returns a new raw payload containing given frames