WASHINGTON — Targeting one of education’s most divisive issues, President Barack Obama on Saturday called for capping standardized testing at 2 percent of classroom time and said the government shares responsibility for turning tests into the be-all and end-all of American schools. Students spend about 20 to 25 hours a school year taking standardized tests, according to a study of the nation’s 66 largest school districts that was released Saturday by the Council of Great City Schools.