GNU `tests/rm/rm2.sh` intermittent

As I understand the reason is the directory structure:

.
./a
./a/1
./a/1/2
./a/0
...

with a/1 having no execute rights. That's why a/1/2 can't be removed. We use remove_dir_all (from std::fs for linux), it tries to iterate the directory recursively and remove everything it finds, but the first time it runs into some error, it stops doing it. So, as I understand, sometimes a/0 is iterated before a/1, and sometimes vice versa, this leads to different results.

The solution could be to implement this iteration manually, not depending on remove_dir_all function. There is already some code that uses walkdir, probably it could be changed to handle both cases.

Start with tests/rm/rm2.sh and reproduce its intermittent failure. Inspect the Rust rm implementation where std::fs::remove_dir_all is used, then review the existing walkdir-based code mentioned in the issue. Done means the test reliably removes the directory structure with a non-executable nested directory and passes consistently.