net: fix protocol number for IPv6 test

The protocol number of ICMP for IPv6 is 58, not 1.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6810093

src/pkg/net/ipraw_test.go

@@ -21,7 +21,7 @@ var icmpTests = []struct {
 	ipv6  bool // test with underlying AF_INET6 socket
 }{
 	{"ip4:icmp", "", "127.0.0.1", false},
-	{"ip6:icmp", "", "::1", true},
+	{"ip6:ipv6-icmp", "", "::1", true},
 }
 
 func TestICMP(t *testing.T) {