In order to test psu that is disconnected from pc or load, you will probably need to use a paper clip to jump the correct pins of 24 pin connector.  It is much easier to test psu connected to a pc motherboard though.  An old ATX desktop pc would do too.
XPS 8930 would not power on, replaced the power supply and all is good. Did not do the self test before replacing. If I do the self test on the removed power supply, will it stay green and spin the fan even though it is not connected to anything in the PC?
XPS 8930, power supply self test work if not connected to PC?