o Code simplification and refactoring:
    - Extracted dummy_origin_circuit_new so it can be used by other test
      functions.
    - Refactor circuit_predict_and_launch_new for readability and testability.
    - Added unit tests for extracted functions.
    - Extracted magic numbers in circuituse.c into defined variables.
    - Refactor circuit_is_available_for_use to remove unnecessary check