Function allow_read_dir_as_file

cpp/src/arrow/filesystem/test_util.h:174–174

- Whether the filesystem allows reading a directory

  virtual bool allow_write_implicit_dir_over_file() const { return false; }
  // - Whether the filesystem allows reading a directory
  virtual bool allow_read_dir_as_file() const { return false; }
  // - Whether the filesystem allows moving a file
  virtual bool allow_move_file() const { return true; }
  // - Whether the filesystem allows moving a directory