Remove a few more cases of mem::uninitialized in the tests
Remove a few more cases of mem::uninitialized in the tests

Somehow I didn't noticed these in PR #1158, probably because they're
only built on Linux.

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