net: re-enable TestDualStack{TCP,UDP}Listener on dragonfly

It looks like the latest DragonFly BSD kernels, at least 4.4 and above,
have finished working on handling of shared IP control blocks. Let's
re-enbale test cases referring to IP control blocks and see what
happens.

Updates #13146.

Change-Id: Icbe2250e788f6a445a648541272c99b598c3013d
Reviewed-on: https://go-review.googlesource.com/19406Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/listen_test.go

@@ -216,9 +216,12 @@ var dualStackTCPListenerTests = []struct {
 // TestDualStackTCPListener tests both single and double listen
 // to a test listener with various address families, different
 // listening address and same port.
+//
+// On DragonFly BSD, we expect the kernel version of node under test
+// to be greater than or equal to 4.4.
 func TestDualStackTCPListener(t *testing.T) {
 	switch runtime.GOOS {
-	case "dragonfly", "nacl", "plan9": // re-enable on dragonfly once the new IP control block management has landed
+	case "nacl", "plan9":
 		t.Skipf("not supported on %s", runtime.GOOS)
 	}
 	if !supportsIPv4 || !supportsIPv6 {
@@ -303,9 +306,12 @@ var dualStackUDPListenerTests = []struct {
 // TestDualStackUDPListener tests both single and double listen
 // to a test listener with various address families, different
 // listening address and same port.
+//
+// On DragonFly BSD, we expect the kernel version of node under test
+// to be greater than or equal to 4.4.
 func TestDualStackUDPListener(t *testing.T) {
 	switch runtime.GOOS {
-	case "dragonfly", "nacl", "plan9": // re-enable on dragonfly once the new IP control block management has landed
+	case "nacl", "plan9":
 		t.Skipf("not supported on %s", runtime.GOOS)
 	}
 	if !supportsIPv4 || !supportsIPv6 {