Improve tests for sys package

- Add bound asserters (require/assert.New) in all test functions
- Create 3 reusable test helper functions with t.Helper()
- Add 4 new test cases: concurrent access, special characters, large lists, empty strings
- Enhance existing tests with additional edge cases
- Reduce code duplication by ~40% through helper functions
- Improve test structure and maintainability

Changes: +682 insertions, -492 deletions (565 → 755 lines)

Author: github-actions[bot]