For Eva Bettencourt, the wait lasted 48 hours. “I just flipped out, like, ‘How does this happen?’” she recalled recently, sitting on her Peabody, Mass., porch. That’s how long it took to find out if the COVID-19 test given to her mother, Arnalda, 81, by a Boston-area nursing home was positive for the disease that has killed more than 70,000 Americans so far.Forty-eight hours –