Onstage pranks are a staple on Kenny Chesney tours, and Megan Moroney might’ve just pulled off the best one ever. During the first of three nights at Massachusetts’ Gillette Stadium on Friday (Aug. 23), Chesney brought opening act Moroney out for a duet performance of “She Thinks My Tractor’s Sexy” — a song they’ve done together several times on the tour. But this time around, Moroney had a little something extra planned. She rolled out onstage on a toy John Deere tractor, dressed like Chesney and waving a No Shoes Nation Flag. Moroney even tucked her hair into a dupe of the star’s signature w…