How to test assert statements with NixOS tests?

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I found a solution that worked. Setup a nixos test where you import the module "${nixpkgs}/nixos/modules/installer/cd-dvd/channel.nix" so you have a channel inside your runner. Then use environment.etc to load config that we want to fail into test runner. We can than execute nixos-rebuild --fast dry-build 2>&1 and check that we get the expected error message.

pkgs.nixosTest {
    name = "test-that-module-fails-right";

    nodes = {
      machine = { config, lib, ... }: {
        imports = [
          # Need nix channel so we can use nixos-rebuild
          "${nixpkgs}/nixos/modules/installer/cd-dvd/channel.nix"
        ];

        environment.etc = {
          "nixos/my-module.nix" = {
            mode = "0600";
            source = ../my-module.nix;
          };

          "nixos/configuration.nix" = {
            mode = "0600";
            text = ''
            {
              imports = [ ./my-module.nix ];
              >>> Set config for my-module that shall fail <<<<<
            }
            '';
          };
        };

      };
    };

    testScript = 
    let 
      expectedError = "my expected error";
    in ''
      start_all()

      with subtest("Check that module fails the right way"):
        status, stdout = machine.execute("nixos-rebuild --fast dry-build 2>&1")
        assert "${expectedError}" in stdout, f"Expected: ${expectedError} but got {stdout}"
    '';
  };
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