I remember going around the A330 line in Toulouse and going aboard one that was still being completed. There was a very old plastic chair where the co-pilot’s seat would evetually be and a worker was sat in it and running tests using the flightdeck screens . I would imagine that the wiring looms are tested before installation, and then power is slowly applied to individual busbars following specific protocols.
Practically a shore supply will be used initially for power up and then probably the apu generator and finally the engine generators