test: support Redox targets #15689

$ cargo check -q -p uu_test --target x86_64-unknown-redox
error[E0432]: unresolved imports `rustix::fs::AtFlags`, `rustix::fs::accessat`
  --> src/uu/test/src/faccessat.rs:20:22
   |
20 |     use rustix::fs::{AtFlags, CWD, accessat};
   |                      ^^^^^^^       ^^^^^^^^ no `accessat` in `fs`
   |                      |
   |                      no `AtFlags` in `fs`