Through the late spring and summer, it looked like America was, slowly but surely, building up its Covid-19 testing capacity. But after July and particularly at the start of September, that progress stalled out. As of September 17, average daily tests were at about 730,000, down from an average of 780,000 in early September and 830,000 in late July, according to the Covid Tracking Project. Meanwhile, the percentage of tests coming back positive, which is used to gauge testing capacity, has remained around 5 percent …