Janet Yellen testified to Congress today about banking, interest rates and the economy. One issue she also addressed: vacancies on the interest-rate setting Federal Open Market Committee. Daniel Tarullo, Federal Reserve governor, resigned last week, leaving President Donald Trump with three openings to fill on the seven-member board. Tarullo was the point man on bank regulation after the 2008 economic crisis, and his resignation gives Trump an opportunity to move faster on his deregulatory agenda.