In testing the Aura.Http package, I have realized that it's much more flexible, testing wise, to pass around file resources (a.k.a. handles, pointers, or streams) than it is to pass around file names. When you do that, you can use a php://memory stream instead of attempting to touch the file system.

He shows how using this method can make it easier to test with a simple bit of refactoring by passing in the "php://memory" stream rather than the actual file reference. The comment (from Chris Hartjes) also suggests an alternative tool for working with tests that need the file system, vfsStream.