By Michael Flaherty and Howard Schneider WASHINGTON (Reuters) – The Federal Reserve is considering turning its annual health check for big banks into one of the tools it could use to prevent a build up of excessive financial risks. If deployed, the modified tests could force banks to retain billions of dollars that otherwise would have gone to shareholders. While the move would help cool off sectors at risk of overheating, it could spark the ire of bank executives, who complain of rising regulatory burdens. Since 2009, the Fed has conducted the so called stress tests on the largest U.S. …