Fix UDPSocket broadcast spec to not use connect (#16165)

AFAICT, calling `connect` is only required because we bind the server to the loopback interface that won't receive broadcasted messages otherwise. For example, on my Linux computer, the loopback interface doesn't have BROADCAST traffic enabled (no MULTICAST).

We can however start a regular UDP server and a UDP client socket with `SO_BROADCAST`, then call `sendto` to send a message to the broadcast address.

Author: ysbaddaden

spec/std/socket/udp_socket_spec.cr

@@ -186,19 +186,31 @@ describe UDPSocket, tags: "network" do
     end
   end
 
-  {% if flag?(:linux) || flag?(:win32) %}
+  {% if flag?(:darwin) %}
+    # Fails since macOS Sequoia (version 15) with EHOSTUNREACH. The ability to
+    # send UDP broadcasts might need extra requirements.
+    pending "sends broadcast message"
+  {% else %}
     it "sends broadcast message" do
-      port = unused_local_tcp_port
+      server = UDPSocket.new(Socket::Family::INET)
+      server.bind("0.0.0.0", 0)
+      addr = Socket::IPAddress.new("255.255.255.255", server.local_address.port)
 
       client = UDPSocket.new(Socket::Family::INET)
-      client.bind("localhost", 0)
       client.broadcast = true
       client.broadcast?.should be_true
-      client.connect("255.255.255.255", port)
-      client.send("broadcast").should eq(9)
+
+      client.send("broadcast", to: addr).should eq(9)
       client.close
+
+      server.read_timeout = 1.second
+      begin
+        message, _ = server.receive
+        message.should eq("broadcast")
+      rescue IO::TimeoutError
+        # Since this test doesn't run over the loopback interface, this test
+        # fails when there is a firewall in use. Don't fail in that case.
+      end
     end
-  {% else %}
-    pending "sends broadcast message"
   {% end %}
 end