If u just want to test to see if they turn on and work for a few seconds then u can, a short burst dry wont be harmfull. If ur so concerned then simply wet the impellar.
Running dry is damaging when done for extended periods of time, heat and friction causes the damage, however a short burst ,especialy if wet, wont cause any damage.
For example, Jay , over at JayzTwoCents, had to salvage a pump, he swaped out the impellars and ran it dry on camera for a second or so to ensure it worked and to show the camera, no harm done.
Again not recommended to run dry, but a few seconds wont hurt especialy if its neccesery for testing purposes like in ur case.