Testing in Go. How to test folder structure using fake folder structure?

I am writing a function to do a recursive deep copy from source folder to destination folder. While my logic works, I would like to write unit tests to ensure all sub-folders and files are getting copied correctly. I believe that using actual files and folders is not a good practice so I was wondering how to stub the file package functions.
Specifically I use file.MatchFiles(path) to get all files and folders in the path provided, then I check if it is a directory or not to do the same recursively.

What's the file package, do you mean os or path/filepath maybe? Anyway, I don't really see what you're aiming it. The functionality of, say, filepath is already guaranteed by tests in the package itself. You don't need to write additional unit tests for functions which are already shipped with such tests. But anyway, to answer your question about good practise: These shipped, official tests work with real access to actual files, so I'ld say that's a valid approach. Makes sense: it's a bad idea to stub the behavior you actually want to test.

So, if you're for example utilizing filepath.WalkDir(), you can write unit tests for your WalkFunc with a custom/mock implementation of FileInfo. If you're using another package, e.g. os, you can also extract your custom logic and test it in a similar manner. If you're still in doubt about the result when all comes together ... well, then do it like in the core libraries did, and use real files for the test.