Federal health officials announced on Wednesday that anyone who wants a coronavirus test may get one if a doctor agrees. But the nation’s testing capacity is still so limited that experts feared clinics and hospitals could be overwhelmed by an avalanche of requests. Under the new criteria, patients who have fevers, coughs or difficulty breathing qualify for diagnostic testing, depending on their doctor’s judgment. But with flu season in full swing, tens of millions of Americans already have respiratory symptoms, and doctors have no quick way to discern who should be tested.