message: Add a test for Clone (for groups)
diff: @@ -199,6 +199,25 @@ pub mod tests {
  })
  }
  + #[test]
  + pub fn test_clone() {
  + with_tmp_file(|file| {
  + file.create_group("a").unwrap();
  + let a = file.group("a").unwrap();
  + assert_eq!(a.name(), "/a");
  + assert_eq!(a.file().unwrap().id(), file.id());
  + assert_eq!(a.refcount(), 1);
  + let b = a.clone();
  + assert_eq!(b.name(), "/a");
  + assert_eq!(b.file().unwrap().id(), file.id());
  + assert_eq!(b.refcount(), 2);
  + assert_eq!(a.refcount(), 2);
  + drop(a);
  + assert_eq!(b.refcount(), 1);
  + assert!(b.is_valid());
  + })
  + }
  +
  #[test]
  pub fn test_len() {
  with_tmp_file(|file| {
  