Add trivial tests for chown and fchownat
Add trivial tests for chown and fchownat

These are mostly to ensure that all the platforms we care about in our
CI can reference these primitives.

asomers/nix,carllerche/nix-rust,asomers/nix,xd009642/nix,nix-rust/nix,geofft/nix-rust,carllerche/nix-rust,MarkusJais/nix-rust,MarkusJais/nix-rust,geofft/nix-rust,nix-rust/nix,carllerche/nix-rust,xd009642/nix,xd009642/nix,geofft/nix-rust,MarkusJais/nix-rust