Clean up the dummy network interface used by UDP tests

It is now composed by a NetworkInterface interface which lets us delete
the methods we don't need.

PiperOrigin-RevId: 351613267

pkg/tcpip/transport/udp/udp_test.go

@@ -1560,33 +1560,17 @@ func TestNoChecksum(t *testing.T) {
 var _ stack.NetworkInterface = (*testInterface)(nil)
 
 type testInterface struct {
-	stack.NetworkLinkEndpoint
+	stack.NetworkInterface
 }
 
 func (*testInterface) ID() tcpip.NICID {
 	return 0
 }
 
-func (*testInterface) IsLoopback() bool {
-	return false
-}
-
-func (*testInterface) Name() string {
-	return ""
-}
-
 func (*testInterface) Enabled() bool {
 	return true
 }
 
-func (*testInterface) Promiscuous() bool {
-	return false
-}
-
-func (*testInterface) WritePacketToRemote(tcpip.LinkAddress, *stack.GSO, tcpip.NetworkProtocolNumber, *stack.PacketBuffer) *tcpip.Error {
-	return tcpip.ErrNotSupported
-}
-
 func TestTTL(t *testing.T) {
 	for _, flow := range []testFlow{unicastV4, unicastV4in6, unicastV6, unicastV6Only, multicastV4, multicastV4in6, multicastV6, broadcast, broadcastIn6} {
 		t.Run(fmt.Sprintf("flow:%s", flow), func(t *testing.T) {