Refactor NixOS test wiring and improve eval efficiency

[roberth]

Issue description

Actionable item from #50301

NixOS could reuse pkgs in pkgs.nixosTest to achieve an evaluation speedup. I also think we should have some tests that exercise the 'normal' modules/misc/nixpkgs.nix functionality, so perhaps all-tests.nix should then have access to two runners: an efficient one and a 'standalone' one.

Excerpt from Ekleog comment:

The objective is:

• When you invoke nix-build nixos/tests/foo.nix, then it'll default to importing ../...
• When you invoke nix-build nixos/release.nix -A tests.foo, then it'll default to, in nixos/release.nix, importing ./.. and using it in tests.foo (and using the same import for all tests if you -A tests, even though that's likely to fail because lots of tests are broken).
• When you invoke nix-build -E '(import <nixpkgs> { overlays = [ ./overlay.nix ]; }).foo.tests, it'll defer to nixosTests.foo (from the same nixpkgs), which will call the test with the same overlay'd nixpkgs.

For the complete discussion, see #50301

[stale]

Thank you for your contributions.

This has been automatically marked as stale because it has had no activity for 180 days.

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[fgaz]

Probably still important, stale-bot

[stale]

I marked this as stale due to inactivity. → More info