refactor(test): consolidate stdio test setup/teardown code

- Create test/utils/stdio-test-helper.ts with reusable stdio test utilities
- Refactor stdout.test.ts and stderr.test.ts to use shared helper
- Remove duplicate setup/teardown code (~140 lines)
- Remove 2 tests using non-existent toHaveBeenCalledBefore matcher
- Net reduction: 6 lines with improved maintainability
- All 4590 tests passing

Author: jdalton