go.net/ipv6: disable tests on non-ipv6 kernels

R=dave, capnm9
CC=golang-dev
https://golang.org/cl/10051046
diff --git a/ipv6/icmp_test.go b/ipv6/icmp_test.go
index 9727829..a7833dd 100644
--- a/ipv6/icmp_test.go
+++ b/ipv6/icmp_test.go
@@ -52,6 +52,9 @@
 	case "plan9", "windows":
 		t.Skipf("not supported on %q", runtime.GOOS)
 	}
+	if !supportsIPv6 {
+		t.Skip("ipv6 is not supported")
+	}
 	if os.Getuid() != 0 {
 		t.Skip("must be root")
 	}