Powerball winner Edwin Castro sent his associate to test drive a $1.5 million Ferrari, seemingly looking to add to his growing collection of luxury cars, RadarOnline.com has learned. While Castro, 32, has kept a low profile since winning over $2 billion in the California Powerball, he has splurged on several purchases, including several prized sports cars. The 32-year-old was spotted at RM Sotheby’s classic car auction on Monday in Culver City. He dressed down in black athletic joggers, a t-shirt, and flip-flops. According to insiders, the Powerball winner arrived before lunch and had an assoc…