For other tests, I don't know how to write them. Somebody will have to teach me how.
And that's nothing to be ashamed of. In fact it's likely a contributing factor to the overall lack of test because few people know how to write them.
I know that I don't have much time in my life, but I want to make something to Widelands community. Not only complaining and writing about bugs on this forum. So I am open to be a tester for new features.
That's great, we all contribute as much as and how we are able. Also, complaints where it is possible to have a fruitful discussion like this are useful too.
re: "corrupted". I see, I wondered if it was mainly a graphics problem showing a messed-up display or similar. Vms usually use their own graphics drivers, and these aren't necessarily made with games in mind. Like you said, it might be an issue with the vm / WMware player, but it could be anything and it's hard to tell.
SDL2? How old is that? Probably none of my games is going with that. (..) If you have any good game which uses SDL2, I can check it working too If it still fails- I will blame the virtual machine, not Widelands
The new version of the library was released roughly two years ago (the old one was 1.2). But then it takes some time before it is packaged and becomes widely enough available for developers to start using it in their projects. And even if a project has switched to SDL2, they have their release schedules so might take a while to get an official release. With that in mind I can't think of any games which have shipped a release build using SDL2. There probably is but I'm not personally aware of any.
If you are up for compiling yourself and like isometric hack-n-slash/action-rpg, then I can recommend FLARE. (Disclaimer: I'm a FLARE contributor) Like Widelands, it's only the development version which use SDL2. The previous release is fun too, but uses an older version of SDL so it would be hard to say whether that's the cause of the problems or not.
I can't host on my virtual machines. I was trying to forward port from host machine, but I failed. I can play only multiplayer games withing my computer: one virtual machine is hosting, second is playing with direct connection. But I can join other people
Similar to how you need to forward ports from your router to your physical machine, you might need to forward the same ports from your host to guest vm. Sounds like what you tried, in which case I guess the WMware documentation should say something about this. I remember fighting with VirtualBox over something like this, but I can't remember if I ever got it to work or not. Apparently, network traffic between vms or host<->vm is fine, but if you want to talk to a vm from the outside you need to configure it.