o Code simplification and refactoring:
    - Refactor circuit_predict_and_launch_new for readability and
      testability. Closes ticket 18873.
    - Extract magic numbers in circuituse.c into defined variables.
    - Refactor circuit_is_available_for_use to remove unnecessary check.

  o Minor features (unit tests):
    - Extract dummy_origin_circuit_new so it can be used by other test
      functions.
    - Add unit tests circuit_predict_and_launch_new.