WASHINGTON - The trafficking of human beings constitutes a ''growth industry'' in the United States, with more than 15,000 people forced into bondage each year as sex slaves or captive laborers, according to testimony before a Senate panel on Wednesday. Although federal laws enacted over the past three years have enabled prosecutors to crack major trafficking rings, two U.S. attorneys and leaders of victim advocacy groups warned that ''modern-day slavery'' is spread across the nation and appears to be growing. ''It seems like we're just touching the tip of the iceberg,'' said Sen. John Cornyn, R-Texas, who presided at Wednesday's...