Thirty-two groups were named Monday to test a new health-care model, called for in the health-care law and designed to improve care for seniors while reducing costs. The groups, which range from Boston-based Partners HealthCare, the largest health-care provider in Massachusetts, to the doctor-led HealthCare Partners of southern Nevada, were selected as the first Medicare accountable care organizations by the Department of Health and Human Services. The organizations are designed to save $1 billion over five years by promoting coordination between doctors and hospitals and ensuring that people with chronic conditions such as diabetes or high blood pressure get the care they need to stay out of the hospital.