Electronic loads are common pieces of test instrumentation. Typically you can program or control them to alter the load current with time so the PSU can be tested for regulation, overshoot, undershoot etc. They're also useful for discharge testing of batteries. You can typically set them for constant current, constant resistance or constant wattage. Asian branded ones start at a few hundred dollars. Hobbyist grade loads are also available for tens of dollars. 

For simple DIY testing a few resistors (possibly switched with a MOSFET) can do a lot. Like this 1\$\Omega\$ 300W resistor. 

[![enter image description here][1]][1]


A full test rack suitable for fully testing a desktop computer power supply could be a [few hundred thousand dollars][2] (link to YT video). 

I've even used automobile headlamp incandescent bulbs. 

  [1]: https://i.sstatic.net/Om2i0.jpg
  [2]: https://youtu.be/b3x28s61q3c