test: skip UDP dualstack test on BSDs

As of right now Linux comes with bindv6only=0 by default but BSDs use
bindv6only=1. It can be changed systemwide with sysctl, but it's not
nice to depend on the environment for running tests, so disable it just
on BSDs.

test/test-udp-ipv6.c

@@ -147,12 +147,19 @@ static void do_test(uv_udp_recv_cb recv_cb, int bind_flags) {
 
 
 TEST_IMPL(udp_dual_stack) {
+#if defined(__DragonFly__)  || \
+    defined(__FreeBSD__)    || \
+    defined(__OpenBSD__)    || \
+    defined(__NetBSD__)
+  RETURN_SKIP("dual stack not enabled by default in this OS.");
+#else
   do_test(ipv6_recv_ok, 0);
 
   ASSERT(recv_cb_called == 1);
   ASSERT(send_cb_called == 1);
 
   return 0;
+#endif
 }