test(nix): Create test suite for Nix hooks

[aRustyDev]

Description

Create comprehensive test suite for all Nix pre-commit hooks.

Requirements

• Create test fixtures (sample Nix files)
• Test success and failure cases for each hook
• Verify error messages are helpful
• Test edge cases and platform compatibility
• Document testing procedures

Test Structure

tests/nix/
├── fixtures/
│   ├── valid-flake.nix
│   ├── invalid-flake.nix
│   ├── darwin-configuration.nix
│   ├── home.nix
│   └── ...
├── test-nix-flake-check.sh
├── test-nix-build-check.sh
├── test-nix-darwin-check.sh
├── test-nix-home-manager-check.sh
├── test-nix-fmt.sh
└── test-nix-lint.sh

Test Cases

• Valid configurations should pass
• Invalid configurations should fail with clear errors
• Missing dependencies should be handled gracefully
• Platform-specific hooks should skip on wrong platforms
• Formatters should produce consistent output
• Linters should catch known anti-patterns

Dependencies

This issue depends on all hook implementations being complete:

• feat(nix): Implement nix-flake-check hook #15-feat(nix): Implement nix-lint hook #20 (all individual hook implementations)

Documentation

• Update README with testing instructions
• Document how to run individual tests
• Provide examples of expected output