Former hostages taking the witness stand at the trial of their allegedIslamic State group captor have described their brutal treatment in chilling detail. Eight former IS hostages have testified so far at the trial of El Shafee Elsheikh, accused of being a member of the notorious kidnap-and-murder cell known as the “Beatles.” But in a quirk of the case — none of the former IS captives has been asked so far to formally identify their alleged captor in court. That’s because the 33-year-old Elsheikh and the other alleged “Beatles” — so-called because of their British accents — took pains to co…