Move logic from `ritest` method to `ritest.py`

More logic from the [ritest method ](https://github.com/py-econometrics/pyfixest/blob/ef1d0449e5af4b1e697c5edb6d496a1e07d456d5/pyfixest/estimation/feols_.py#L2215) should be moved to [ritest.py](https://github.com/py-econometrics/pyfixest/blob/master/pyfixest/estimation/ritest.py).