While flight simulation is an important part of pilot training, for the RAF aptitude test, you will not need any previous knowledge or experience of flying.
This means that using a DCS with a joystick as practice will not help you perform better on the RAF aptitude test.
With this being said, however, previous recruits have found it useful to practice for any flight simulations assessments using a DCS and a joystick, so it might be helpful later in the training process.