func (fs FileSystem) MkdirAll(dirname string) error {
	fs.testDriver()
	dirname = filepath.FromSlash(dirname)
	return fs.drv.MkdirAll(dirname)
}
MkdirAll creates directories and their parents, if needed.
package fs
import (
	"errors"
	"os"
	"path/filepath"
)
var ErrNoDriver = errors.New("fs: nil driver")
type FileSystem struct {
	drv Driver
}
type Driver interface {
	MkdirAll(dirname string) error
	ReadDir(dirname string) ([]FileInfo, error)
	ReadFile(filename string) ([]byte, error)
	Stat(filename string) (FileInfo, error)
	Symlink(oldname, newname string) error
	WriteFile(filename string, data []byte, perm os.FileMode) error
}
func New(drv Driver) FileSystem {
	return FileSystem{drv}
}