nixosTests: drop nr_tests and failed_tests variables
nixosTests: drop nr_tests and failed_tests variables

With the tests now bailing out early on a failing subtest, we don't need
to keep a list of failed tests, or the number of total tests

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