Is there a way to test a module without uploading them first?
Sure, all you need to do is run it on your computer, if you need to check connectivity then download the file to your friend and you can both connect to the vassal server it doesn’t need to be on site for you to connect with others who have your mod. There is also a way to have your machine act as a server and let other testers connect to your machine, but its more involved and you will need to modify your router to allow connections to your machine.