Fix #111

[JRF63]

The root cause here is that this test should be deleting t and not the deeply nested sub-directory under it.

The error is because Rust's implementation of remove_dir_all is doing recursion which doesn't close the directory handles until it reaches the deepest subdir. The implementation of rm can delete t because it opens only one handle so it won't get the "Too many open files" error. It does comes with the tradeoff of having to cache the visited files though.