The Food and Drug Administration on Saturday authorized emergency use of a new and inexpensive saliva test for Covid-19 that could greatly expand testing capacity. The new test, which is called SalivaDirect and was developed by researchers at the Yale School of Public Health, allows saliva samples to be collected in any sterile container. It is a much less invasive process than the nasal swabs currently used to test for the virus that causes Covid-19, but one that has so far yielded highly sensitive and similar results. The test, which also avoids a key step that has caused shortages of chemical reagents used in other tests, can run approximately 90 samples in fewer than three hours in a lab, although the number can be greater in big labs with automation.