Test that a global IPv6 subnet is public and never counts as the local network (issue 154, ADR 0340)

The review asked for the IPv6 side of the rule that local-network cannot open a port to the internet.

internal/catalogue/localnetwork_test.go

@@ -175,3 +175,21 @@ func TestAnAdoptedNodeOpensTheLocalNetworkPortToTheMeshOnly(t *testing.T) {
 		t.Fatalf("openings %v", out)
 	}
 }
+
+// A global unicast IPv6 subnet on a link facing outside is public, and never counts: the IPv6 side of the
+// rule that `local-network` cannot open a port to the internet (ADR 0340 §2).
+func TestAGlobalIPv6SubnetIsPublicAndNeverCounts(t *testing.T) {
+	got := JudgeLocalNetworks([]AttachedNetwork{{Link: "eth0", Network: "2001:db8:1234::7/48"}}, []string{"eth0"}, "wg-mesh")
+	if len(got) != 1 {
+		t.Fatalf("judged %+v", got)
+	}
+	if got[0].Counts {
+		t.Fatalf("the global IPv6 subnet %s counts as a local network", got[0].Network)
+	}
+	if got[0].Network != "2001:db8:1234::/48" || !strings.Contains(got[0].Why, "public, never counted") {
+		t.Fatalf("judged %s with why %q, want it named public and never counted", got[0].Network, got[0].Why)
+	}
+	if n := CountedLocalNetworks(got); len(n) != 0 {
+		t.Fatalf("counted %v", n)
+	}
+}