Add TESTING.md with complete documentation for the Ralph test suite:
- Quick start commands for running tests
- Test organization and directory structure
- BATS syntax guide with examples from the codebase
- Test helper API documentation (assertions, mocks, fixtures)
- Coverage requirements and kcov limitations
- CI/CD integration with GitHub Actions pipeline
- Troubleshooting guide for common issues
- Appendices with quick reference and patterns
Covers all 276 tests across 11 test files.
