Switch dummy FTP server to use IP address instead of hostname

Using a hostname does cause an occasional leaking thread in the Ruby socket
library when using Ruby 3.4.1. Switching to the IP address mitigates the issue.

library/net-ftp/fixtures/server.rb

@@ -9,7 +9,7 @@ class DummyFTP
     attr_reader :server_port
 
     def initialize
-      @hostname = "localhost"
+      @hostname = "127.0.0.1"
       @server = TCPServer.new(@hostname, 0)
       @server_port = @server.addr[1]