test,openbsd: use RETURN_SKIP in UDP IPv6 tests

IPv6-IPv4 dual stack is not supported on OpenBSD.

Upstreamed from the OpenBSD Ports Collection.

PR-URL: https://github.com/libuv/libuv/pull/1797
Reviewed-By: Aaron Bieber <deftly@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

test/test-udp-ipv6.c

@@ -174,6 +174,8 @@ TEST_IMPL(udp_dual_stack) {
 #if defined(__FreeBSD__) || defined(__FreeBSD_kernel__) || defined(__NetBSD__)
   if (!can_ipv6_ipv4_dual())
     RETURN_SKIP("IPv6-IPv4 dual stack not supported");
+#elif defined(__OpenBSD__)
+  RETURN_SKIP("IPv6-IPv4 dual stack not supported");
 #endif
 
   do_test(ipv6_recv_ok, 0);