Air Inc. provides a useful prep. If you like to chair fly for a sim event, this is an opportunity to chair fly the Delta interview. Try to get a retired Delta Capt. for the session. The former Delta folks know the Delta ideas on culture and ethics better than anyone else.
Quite a few applicants are doing well on the testing and flubbing the interview. Just like anything else, practice makes perfect.
There is not a practice tutorial for the actual Delta tests, but the gouge provides good ideas for study and I had my wife give me random numbers & letters to memorize in the week leading up to the interview just to practice short term memory skills.
You will find the interview is very relaxed, but thorough. Be very careful with your application, be very VERY honest. Delta is a great place to be - good luck.