I built a test jig, which turned out to be a universal jig for Arduino Uno form factor PCBs without headers (when all pins are populated).
I ordered PCBAs with SMD components only to save cost and be able to customize for Arduino or Feather headers. To test the boards without adding connectors the jig comes into play. I used an old shield PCB and drilled out the holes to fit 1mm pogo pins. The pins soldered into IC socket rows and stuck through the PCB r proper alignment. Now I can stick the jig into an Arduino and place the shield to be tested on top and press down.
A mechanical test fixture to clamp down the board stack would be helpful, but for now it is only 25 boards to test. No really mass production yet. A crude clamp manually attached helps for longer test runs for now.