Function allow_write_file_over_dir

cpp/src/arrow/filesystem/test_util.h:169–169

- Whether the filesystem may allow writing a file "over" a directory

  virtual bool have_implicit_directories() const { return false; }
  // - Whether the filesystem may allow writing a file "over" a directory
  virtual bool allow_write_file_over_dir() const { return false; }
  // - Whether the filesystem may allow writing a directory "over" a file,
  //   for example copying file "A" to "B/C" while "B" exists and is a file.
  virtual bool allow_write_implicit_dir_over_file() const { return false; }