Add unittest

Add unittest
[   32s] + ./walkdir
[   32s]
[   32s] running 44 tests
[   32s] test tests::recursive::contents_first ... ok
[   32s] test tests::recursive::empty ... ok
[   32s] test tests::recursive::empty_file ... ok
[   32s] test tests::recursive::empty_file_follow ... ok
[   32s] test tests::recursive::empty_follow ... ok
[   32s] test tests::recursive::many_dirs ... ok
[   32s] test tests::recursive::many_files ... ok
[   32s] test tests::recursive::filter_entry ... ok
[   32s] test tests::recursive::max_depth_0 ... ok
[   32s] test tests::recursive::many_mixed ... ok
[   32s] test tests::recursive::max_depth_1 ... ok
[   32s] test tests::recursive::max_depth_2 ... ok
[   32s] test tests::recursive::min_depth_1 ... ok
[   32s] test tests::recursive::min_depth_2 ... ok
[   32s] test tests::recursive::min_max_depth_diff_0 ... ok
[   32s] test tests::recursive::min_max_depth_diff_1 ... ok
[   32s] test tests::recursive::min_max_depth_diff_nada ... ok
[   32s] test tests::recursive::nested ... ok
[   32s] test tests::recursive::one_dir ... ok
[   32s] test tests::recursive::nested_small_max_open ... ok
[   32s] test tests::recursive::one_dir_one_file ... ok
[   32s] test tests::recursive::one_file ... ok
[   32s] test tests::recursive::regression_skip_current_dir ... ok
[   32s] test tests::recursive::same_file_system ... ok
[   32s] test tests::recursive::send_sync_traits ... ok
[   32s] test tests::recursive::siblings ... ok
[   32s] test tests::recursive::skip_current_dir ... ok
[   32s] test tests::recursive::sort_by ... ok
[   32s] test tests::recursive::sort_by_file_name ... ok
[   32s] test tests::recursive::sort_by_key ... ok
[   32s] test tests::recursive::sort_max_open ... ok
[   32s] test tests::recursive::sym_dir_follow ... ok
[   32s] test tests::recursive::sym_dir_nofollow ... ok
[   32s] test tests::recursive::sym_dir_self_loop_io_error ... ok
[   32s] test tests::recursive::sym_file_follow ... ok
[   32s] test tests::recursive::sym_file_nofollow ... ok
[   32s] test tests::recursive::sym_file_self_loop_io_error ... ok
[   32s] test tests::recursive::sym_noloop ... ok
[   32s] test tests::recursive::sym_root_dir_follow ... ok
[   32s] test tests::recursive::sym_root_file_follow ... ok
[   32s] test tests::recursive::sym_root_dir_nofollow ... ok
[   32s] test tests::recursive::sym_root_file_nofollow ... ok
[   32s] test tests::recursive::sym_loop_detect ... ok
[   32s] test tests::recursive::sym_self_loop_no_error ... ok
[   32s]
[   32s] test result: ok. 44 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.06s