Enhance test suite with comprehensive unit tests and CI compatibility

- Add comprehensive_unit_tests.jl: 400+ tests covering all Mera functions
- Add comprehensive_unit_tests_simple.jl: Focused Mera-specific testing
- Update runtests.jl: Integrate new comprehensive test suites
- Implement CI detection for notification functions (bell, notifyme)
- Add robust error handling and edge case testing
- Ensure CI compatibility: skip audio/email functions on headless systems

Author: ManuelBehrendt