- Make tests/lxd.nix use NixOS's lxdMeta & lxdImage to avoid relying on
3rd party containers such as Alpine Linux for testing purposes.
- Merge tests/lxd-image.nix into tests/lxd.nix, since now both have a
similar structure.
- Extract duplicated inline LXD configuration into a separate file,
- Add passthru.lxd-nftables & passthru.lxd-image-server.