Ofborg passthru.tests nixos tests with Darwin workaround should be reworked once underlying issue is fixed

[mattpolzin]

Describe the bug

A growing number of PRs (see Additional Context below) are working around ofBorg CI failures when executing nixos tests on Darwin platforms. The underlying issue is tracked in #294725. Once that issue has been addressed, we can undo the workaround in these PRs (go back to simply referring to nixos tests unconditionally).

The tests fail with an error similar to:

error: a 'x86_64-linux' with features {} is required to build '/nix/store/iz6wl6z51wgh2rpp245jgx002hcz55vg-Xresources-Xft.drv', but I am a 'x86_64-darwin' with features [...]

The workaround tends to look roughly like the following:

tests = lib.optionalAttrs stdenv.buildPlatform.isLinux {
      # Ref: https://github.com/NixOS/nixpkgs/issues/345825
      test = nixosTests.terminal-emulators.rio;
    };

Expected behavior

Nixos tests should run (or at least gracefully skip themselves) on darwin architectures without explicitly avoiding them with optionalAttrs.

Additional context

In addition to the originally linked PR (344751), where a follow-up has been opened to skip the test, I know of at least one other location (Kitty) where the problem has been worked around -- this was where I found my precedent to suggest the workaround myself. A quick grep of rg 'tests = lib.optionalAttrs' turned up another location (zutty) so I suspect a fuzzier search may turn up a few others.

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[mattpolzin]

Thinking about this a bit more, it seems possible to me that it isn't all nixos tests that fail on Darwin but rather specific ones that rely on derivations that only support Linux. That's just a guess, but it occurred to me I may have overstated things by saying that nixos tests are failing on Darwin as a blanket statement.

[wegank]

Duplicate of #294725

[mattpolzin]

I'll repurpose this ticket to track the necessary cleanup work after the underlying issue (#294725) is fixed.