1.6.0_beta: Update tests for Simulation cherenkov_light_engine default to 'Greisen'

- Adjusted existing tests to reflect that 'Greisen' is now the default value for the cherenkov_light_engine field.
- Added new tests to verify that 'Default' is replaced by 'Greisen' during simulation configuration.
- Included validation tests to ensure that only valid values ('Greisen', 'Gaisser-Hillas') are accepted for the cherenkov_light_engine field, and that invalid values raise appropriate errors.