tests.overriding: structure tests as an attribut set

[ShamrockLee]

Description of changes

This PR restructures the exposed passthru.tests of tests.overriding from a list to an attribute set, making individual tests accessible by their names.

This is extracted from #330010 and implemented with currently available library functions. Whether to add lib.concatMapStringAttrsSep is still under discussion.

Cc: @roberth

Things done

• Built on platform(s)
  • x86_64-linux (no rebuild)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[ShamrockLee]

Result of nixpkgs-review run on x86_64-linux 1

1 package blacklisted:

• nixos-install-tools

[roberth]

@ofborg build tests.overriding

[roberth]

Thanks!