cam-service: Add unit test for cam_socket module

Add unit test for cam_service.
- Use CTest and CUnit to add the test framework for cam_service.
- Add test cases for cam_socket module.
- Update docker file to add the CUnit installation.
- Update README to add the CUnit dependency.
- Add unit test into CI.

Signed-off-by: Yanqin Wei <yanqin.wei@arm.com>