Main Stem vet Mary Testa will star in a new tuner, “I’ll Be Damned,” set for an Off Broadway run this fall.Testa (“Guys and Dolls,” “Xanadu”) appears alongside Kenita R. Miller (“The Color Purple”) and Kurt Robbins in the musical, which has music by Rob Broadhurst and lyrics by Brent Black, with a book co-written by the two creatives. Storyline centers on a comicbook geek who gets an offer from the devil as he searches for a true friend. Broadhurst and Black are recent grads of New York U., where they penned the tuner as part of the Tisch Graduate Musical Theater Writing Program. Jaradoa Theater produces “Damned,” which is helmed by April Nickell. Show begins perfs July 1 at the Dimson Theater at the Vineyard.