The True Test of Stamina on the Road to the Derby   The $250,000 Grade 2 Remsen Stakes is arguably the most demanding 1 1/8-mile test for two-year-old colts in the late racing season. Held for the last time in the  Aqueduct Racetrack, this race is a critical barometer for identifying genuine routers, horses capable of handling the punishing 1 ¼ miles of the Kentucky Derby.