nixos/tests: fix testDriver reference in runInMachine function
nixos/tests: fix testDriver reference in runInMachine function

In a previous commit I broke this as there is no longer one testDriver
but only a function to generate one based on some QEMU inputs.

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