net: disable "udp" to IPv6 unicast address loopback test on dragonfly

Disable the "udp" to IPv6 unicast address on the loopback interface
test under DragonFly BSD. This currently returns a local address of
0.0.0.1, rather than an IPv6 address with zone identifier.

Update #7473

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh
CC=golang-codereviews
https://golang.org/cl/71500044

src/pkg/net/udp_test.go

@@ -201,6 +201,10 @@ func TestIPv6LinkLocalUnicastUDP(t *testing.T) {
 		{"udp", "[" + laddr + "%" + ifi.Name + "]:0", false},
 		{"udp6", "[" + laddr + "%" + ifi.Name + "]:0", false},
 	}
+	// The first udp test fails on DragonFly - see issue 7473.
+	if runtime.GOOS == "dragonfly" {
+		tests = tests[1:]
+	}
 	switch runtime.GOOS {
 	case "darwin", "dragonfly", "freebsd", "openbsd", "netbsd":
 		tests = append(tests, []test{