You can test using the single psu, by taking a paperclip and shorting the green wire on the psu 24 pin connector to any of the black wires on that same connector. Another cheap option is THIS connector which simply plugs into your psu. A second psu is not necessary.

All you have to do is unplug everything from the psu except for the pump, and power up after doing the short, or that bypass, and leak test the system.

Another tip is to try to make sure you have no twists in your tubing while installing it. Twisting can lead to easier kinking and problems.

I also see no problem with leak testing inside the case, as long as you take the proper precautions, as Fragger already suggested.

+1 on the D5. It is a good pump that should last.