test/socket: rescue Errno::ENOBUFS in test_udp_server (ruby#16553)

The server thread's msg_src.reply can raise Errno::ENOBUFS
("No buffer space available") transiently on macOS CI. This
exception was not caught, propagating through th.join in the
ensure block and failing the test.

Rescue it in the server thread so the client side times out on
IO.select, raises RuntimeError, and hits the existing omit path.

Author: k0kubun

test/socket/test_socket.rb

@@ -418,12 +418,16 @@ def test_udp_server
 
         ping_p = false
         th = Thread.new {
-          Socket.udp_server_loop_on(sockets) {|msg, msg_src|
-            break if msg == "exit"
-            rmsg = Marshal.dump([msg, msg_src.remote_address, msg_src.local_address])
-            ping_p = true
-            msg_src.reply rmsg
-          }
+          begin
+            Socket.udp_server_loop_on(sockets) {|msg, msg_src|
+              break if msg == "exit"
+              rmsg = Marshal.dump([msg, msg_src.remote_address, msg_src.local_address])
+              ping_p = true
+              msg_src.reply rmsg
+            }
+          rescue Errno::ENOBUFS
+            # transient OS error on macOS CI, let client timeout and omit
+          end
         }
 
         ifaddrs.each {|ifa|