net: disable TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion [sic] on DragonflyBSD

Like OpenBSD, DragonflyBSD does not do IPv4-in-IPv6 mapping.

For #52264.

Change-Id: Id41606e75652e684f2e0e98c6459156641aec9b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/420775
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: rsc

src/net/udpsock_test.go

@@ -633,7 +633,9 @@ func TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
 	}
 
 	switch runtime.GOOS {
-	case "openbsd":
+	case "dragonfly", "openbsd":
+		// DragonflyBSD's IPv6 sockets are always IPv6-only, according to the man page:
+		// https://www.dragonflybsd.org/cgi/web-man?command=ip6 (search for IPV6_V6ONLY).
 		// OpenBSD's IPv6 sockets are always IPv6-only, according to the man page:
 		// https://man.openbsd.org/ip6#IPV6_V6ONLY
 		t.Skipf("skipping on %v", runtime.GOOS)