nixosTest: make modular

[roberth]

Why

• Generated docs
• Type checking
• Overrides for interactive use
• Reusable test fixtures with imports
• Replace test logic with disabledModules
• Work towards generalized test matrix feature (nixosTest matrix (prev also modularity, option docs) #176557)
• Some other fixes

Closes #171396

What

• Move testing-python.nix logic and build-vms.nix logic into modules
• Rewrite the node numbering logic
• Write docs

Extracted from #176557

Hydra: 4299c17 https://hydra.nixos.org/eval/1781169?compare=1780016

First nixos-unstable: https://hydra.nixos.org/eval/1782482

"removed" attributes

• ec2: meta.broken now applies to test runner
• fcitx: meta.broken now applies to test runner
• gocd-agent: meta.broken now applies to test runner
• hadoop: rename the attr path to include system
• hocker-fetchdocker: meta.broken now applies to test runner
• installed-tests.xdg-desktop-portal: meta.broken now applies to test runner
• mariadb, mysql: now runs on both system types, changing the attr path
• pulseaudio: now runs on both system types, changing the attr path
• shadowsocks: now runs on both system types, changing the attr path
• zigbee2mqtt: name has already been added in master. merge will fix this.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 22.11 Release Notes (or backporting 22.05 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
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[roberth]

Ofborg ran out of disk space.

@ofborg eval

[Atemu]

This broke kernel passthru tests:

$ nix-instantiate -A linux.passthru.tests --system x86_64-linux --show-trace
error: attempt to call something which is not a function but a set

       at /Users/atemu/Projects/nixpkgs/pkgs/os-specific/linux/kernel/generic.nix:217:11:

          216|       };
          217|     in [ (nixosTests.kernel-generic.testsForKernel overridableKernel) ] ++ kernelTests;
             |           ^
          218|   };

[roberth]

The way both nixosTests and nixos/release.nix load and transform the tests is incredibly ad hoc and fragile.
It appears that nixosTests now tries to invoke the test according what we might call the standalone import ./make-test-python.nix pattern, considering that testsForLinuxPackages appears to receive { pkgs = ...; system = ...; } as its argument. Both discoverTests and handleTest pass these arguments, but perhaps discoverTests is at fault. I really can't tell though, because of all the "legacy" that this code needs to support.

Ideally we'd standardize the solution for both the test matrix "problem" and the package testing problem in one go. That could be #176557 or something similar.

[roberth]

Reading through the changes, e77913a may be the commit to blame, but reverting it is likely to break some other tests' glue.