betaboon: so, your needed file is installer.nix. there you would need to:
1. add support for new `bootLoader` value (say "refind"), like for example https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/installer.nix#L45
2. also use EFI mode for it for obvious reasons, https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/installer.nix#L74
3. check that bootloader configuration is created properly here https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/installer.nix#L134
4. use the same bootloader for the machine that runs the installer, here https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/installer.nix#L269 . also add refind into `extraDependencies`
5. finally, just copy for example GRUB EFI test https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/installer.nix#L365 and https://github.com/NixOS/nixpkgs/blob/master/nixos/tests/installer.nix#L444 for your bootloader
6. you can try it with `nix-build -A tests.installer.simpleUefiGrub.x86_64-linux nixos/release.nix`