Riding the ever-growing sensation of “This Is Spinal Tap” (1984) and freshly inspired by a chance encounter with an amateur production of “Annie Get Your Gun,” Christopher Guest cemented his filmmaking voice in the mid-1990s. With “Waiting For Guffman” in 1996 and then “Best In Show” (20 years old this week), the “Saturday Night Live” alum strung together an unlikely wave of bone-dry ensemble comedies.