It felt like a rock ‘n’ roll nightmare to watch. The Tritons, a band, bring their girlfriends to a farmhouse near Toronto for a retreat where they plan to write music in the barn, which has been converted into a studio. Jon Mikl Thor leads the cast of unknowns as the singer who resembles David Lee Roth. “Rock ‘n’ Roll Nightmare” (1987), also called “The Edge of Hell,” combines cabin-in-the-woods horror with 198…