tests/sys_atomic_utils_unittests: Extend test coverage
tests/sys_atomic_utils_unittests: Extend test coverage

OlegHahm/RIOT,OlegHahm/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,miri64/RIOT,jasonatran/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,miri64/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,miri64/RIOT