nixos/tests/openarena: run real openarena clients
nixos/tests/openarena: run real openarena clients

The old Quake3 NixOS test was removed in
50ea99cbc18d3f480a773de5250b4ef9c7f6d514 which served as a nice demo to
showcase what NixOS tests are capable of. This commit adds the same
functionality to run real openarena clients.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs