Move NixOS systems tests to `nixos/tests`.

nixos/system-tests.nix

nixos/tests/exercise-storage.py

nixos/tests/get-passes.py

nixos/tests/node.pem

nixos/tests/private-storage.nix

nixos/tests/probeuser_ed25519

nixos/tests/probeuser_ed25519.pub

nixos/tests/run-client.py

nixos/tests/run-introducer.py

nixos/tests/stripe-api-double.py

nixos/tests/tahoe.nix