The state’s former inspector general is joining the Pioneer Institute, a Boston-based think tank. Gregory W. Sullivan will be the institute’s research director, focusing on a variety of public policy issues for the conservative-leaning institute. Sullivan served two five-year terms as Massachusetts inspector general, a position appointed by the governor. The office was created to look into fraud, waste, and abuse in state government. Sullivan, a Democrat, previously served for 17 years in the Massachusetts House of Representatives. He replaces Steve Poftak, who left the Pioneer Institute last year for a position at Harvard’s John F. Kennedy School of Government.