nixos/tests/shadow: Fix linting errors
nixos/tests/shadow: Fix linting errors

Linter errors reported:

  6:32 f-string is missing placeholders
  7:26 f-string is missing placeholders
  8:32 f-string is missing placeholders
  30:32 f-string is missing placeholders
  31:26 f-string is missing placeholders
  32:32 f-string is missing placeholders
  48:32 f-string is missing placeholders
  49:26 f-string is missing placeholders
  50:32 f-string is missing placeholders
  76:32 f-string is missing placeholders
  77:26 f-string is missing placeholders
  78:32 f-string is missing placeholders

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@nix.build>

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