This work details the migration of the preCICE Fluent adapter from version 2 to version 3, alongside the development of a comprehensive unit testing framework using C-based mocking libraries. The migration ensures full compatibility with the latest preCICE framework, thereby enhancing performance and integration capabilities for multi-physics simulations. In parallel, the introduction of unit tests using advanced mocking techniques has enabled rigorous validation of the adapter's functionality, error handling, and interface stability. The combined efforts in upgrading and testing not only improve reliability and maintainability but also lay the groundwork for further advancements in coupled simulation environments.