Well, filesystem is something you don't want to do over the internet when running your tests right? If you don't have internet or AWS is down your tests will fail, while the code might work just fine!
So instead you need to trust the Filesystem tests and test your file upload stuff with a local driver.
You can however write a test for this, but that requires you to have a real s3 bucket and a solid internet connection of course. It's not impossible, but it shouldn't be needed because the storage component is Laravel is already battle tested ;)