nixos/tests/systemd: Slightly changed IP accounting message

          assert "0B read, 0B written" not in output

      with subtest("systemd per-unit accounting works"):

          assert "IP traffic received: 84B sent: 84B" in output_ping

          assert "IP Traffic: received 84B, sent 84B" in output_ping

      with subtest("systemd environment is properly set"):

          machine.systemctl("daemon-reexec")  # Rewrites /proc/1/environ