tests_ls.rs: remove nix

[oech3]

No description provided.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/tail-n0f (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.

[cakebaker]

Thanks!