Despite the great medical advances of the past half-century, cancer has proved an elusive enemy; for many strains of the disease, survival rates are scarcely better now than they were decades ago. So every new discovery is met with enthusiasm, and that’s certainly the case for a new blood test announced earlier this week by Boston scientists, who are teaming with Johnson & Johnson to market a test that will help doctors determine what cancer cells are doing and how best to attack them. The new test, using only a small sample of blood, is said to be so sensitive that it will measure one cancer cell among a billion healthy ones.