tstest/natlab: be more paranoid about IP versions from gvisor

I didn't actually see this, but added this while debugging something
and figured it'd be good to keep.

Updates #13038

Change-Id: I67934c8a329e0233f79c3b08516fd6bad6bfe22a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/natlab/vnet/vnet.go

@@ -234,7 +234,7 @@ func (n *network) handleIPPacketFromGvisor(ipRaw []byte) {
 			ipRaw,
 			layers.LayerTypeIPv6, gopacket.Lazy)
 	default:
-		return
+		panic(fmt.Sprintf("unexpected IP packet version %v", ipVer))
 	}
 	flow, ok := flow(goPkt)
 	if !ok {