kmdreko's hint seems to work. Oracle now owns RC<RefCell>

pub struct Oracle {
    source: Rc<RefCell<dyn WisdomSource>>,
}

impl Oracle {
    pub fn new() -> Oracle {
        Oracle {
            source: Rc::new(RefCell::new(Magic8Ball))
        }
    }

    pub fn ask(&self, question: &String) -> String {
        self.source.borrow_mut().ask(question)
    }
}

Then at unit test, I will holdon to RC of RefCell of the mock and inject clone of that RC downcasted to Oracle

#[test]
fn test_sage() {
    let mockup_rc = Rc::new(RefCell::new(MockWisdomSource::new()));
    mockup_rc.borrow_mut().expect_ask().times(1).return_const("42".to_string());

    let source = Rc::clone(&mockup_rc) as Rc<RefCell<dyn WisdomSource>>;        
    let sage = Oracle{source};
    
    let answer = sage.ask(&"What is the meaning of life?".to_string());

    assert_eq!(answer, "42");
    
    // Check that the mockup was called
    mockup_rc.borrow_mut().checkpoint();
}