the DTM(Direct Test Mode) is defined by the Bluetooth specification, see BLUETOOTH SPECIFICATION Version 5.0 | Vol 6, Part F.
Hence, any tester that is compliant with the Bluetooth DTM specification can be used to test our nRF5x devices. You only need to flash the Direct Test Mode example to the nRF device and modify the UART pin numbers that is connected to the DTM tester if you're testing a custom board.
