test(nix): Create test suite for Nix hooks

## 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:
- #15-#20 (all individual hook implementations)

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