It has been one of the most stubborn problems in education: With 50 states, 50 standards and 50 tests, how could anyone really know what American students were learning, or how well? At a dinner with colleagues in 2009, Mitchell Chester, Massachusetts’s commissioner of education, hatched what seemed like an obvious answer — a national test based on the Common Core standards that almost every state had recently adopted. Now Dr. Chester finds himself in the awkward position of walking away from the very test he helped create. On his recommendation, the State Board of Education decided last week that Massachusetts would go it alone and abandon the multistate test in favor of one to be developed for just this state. The move will cost an extra year and unknown millions of dollars.