message: test(nydus-image): remove unneeded `tree` dependency in unit test
  Use `ls -R` instead, to avoid using `tree`.
  fix
diff: @@ -979,11 +979,11 @@ pub mod tests {
  println!(
  "lower dir:\n {}",
  - exec(&format!("tree -a {:?}", lower_dir), true).unwrap()
  + exec(&format!("ls -R {:?}", lower_dir), true).unwrap()
  );
  println!(
  "merge dir:\n{}",
  - exec(&format!("tree -a {:?}", merge_dir), true).unwrap()
  + exec(&format!("ls -R {:?}", merge_dir), true).unwrap()
  );
  // Diff build lower layer
  