Here is what you would have to do to test for COVID-19

Take the sample,
kill everything,
separate out the DNA and RNA,
recombine and lengthen them,
put them through a DNA analyzer (one type uses a laser to burn each micro drop that is squeezed out of a tiny tube, and the reflected light from each burned piece gives you a letter code... sorta)

And then compare that DNA signature with that of COVID-19.

It just ain't happening.

So, most tests for viruses test for a lack of some chemical (what the virus ate) or the excess of some chemical (what the virus shat out)

And both of these are really poor, as there are millions of ways to get a false positive.

And false negatives can easily be gotten just by swabbing the wrong area.