sums: skip broken tests subdir to fix parallel build

[MiniHarinn]

ZHF: #516381 (Part of)

Hydra Failure (Click the banner to go to Hydra build report):

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[MiniHarinn]

nixpkgs-review result

Generated using nixpkgs-review-gha

Command: nixpkgs-review pr 517936
Commit: 4a9e379ff1668216520011f437ba032fd89261c2 (subsequent changes)
Merge: 23dcc1d66677aa26dbb7b8b11c1f67f8ee9b30dc

Logs: https://github.com/MiniHarinn/nixpkgs-review-gha/actions/runs/25539745137

---

x86_64-linux

✅ 1 package built:

• sums

---

aarch64-linux

✅ 1 package built:

• sums

---

x86_64-darwin (sandbox = relaxed)

❌ 1 package failed to build:

• sums

---

aarch64-darwin (sandbox = relaxed)

❌ 1 package failed to build:

• sums

[MiniHarinn]

The Darwin failure seems unrelated and is a cascading failure from another failing package.