mkdir(recursive: true) behaves wrong for existing directories

[heinebold]

Normally, when you mkdir('/this/already/exists', recursive: true) a path that already exists, php does not complain (although it does if you don't specify "recursive"). THis means that a recursive mkdir is a simple one-line way to ensure the directory exists, without having to wrap it in manual checks.

In vfsStream though, it will say "mkdir(): Path vfs://this/already/exists exists", which means you can't correctly test the behavior of something that relies on mkdir(recursive: true).