tests: Fix the start of the daemon
tests: Fix the start of the daemon

- Make sure that it starts even without the `nix-command` xp feature
- Fail if it doesn’t manage to start

This fixes a 30s wait for every test in `init.sh` as the daemon couldn’t
start, but the code was just waiting 30s and continuing as if everything
was all right.

NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix