Motivation for this change

While porting some tests to use the new python test-driver, some of the subtests failed. Finding the exception and the respective subtest was annoyjing for me, since there is a lot of other output printed to the terminal. At a NixOS meetup @0x4A6F approached me and asked me if I could make some changes in the test-driver, such that the failed tests are shown in a clear way.

This was the output before the changes:

1 out of 2 tests succeeded
cleaning up
killing machine (pid 6)

This is the new output:

1 out of 2 tests succeeded
The following tests have failed:
 - Test "root fs is fsckd" failed with error: "Failing for demonstration purposes"
cleaning up
killing machine (pid 6)

I'm sure that there are a few more improvements that can be done to make the output uncluttered. Feel free to comment your ideas.

@tfc @flokli @worldofpeace #72828

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• [x ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.