All that the power switch does is to momentarily short out the appropriate pins on the motherboard front panel header. The Reset switch does exactly the same thing to the reset pins on the header. Therefore you can swap the Reset and Power switch connectors to test the correct operation of either. In this configuration the Reset switch would turn on the computer.
Alternatively you can just momentarily short out the power switch pins on the header with a screwdriver or something similar but be very careful to safeguard against unintentional short circuits.