As countries around the world try to get their economies back on track, the number of tests for coronavirus that test positive has become the indicator to watch. 5% is the threshold to reopen safely; 10% is disturbing, 20% outrageous. In Mexico, the rate stands at 50%. The high numbers are easy to explain, although not so easy to solve. The Latin American country has stubbornly declined to run large-scale tests and instead examines only those who…