I agree with Larry, use a battery. I hooked up a charger to my battery to keep it fresh. You can't do all the operational tests but you can at least check that each item is connected and powering up correctly and many items you can at least see are working in static configuration. I dunno about the GRT, but if sensors were not connected to the Engine Pod on my plane, there would be no reading as opposed to a static reading when connected. Tach, for instance, read "--" without the sensor and "0" with the sensor connected. Before I permanently installed the GPS antennae, I ran them outside the hangar and was able to test the database and flight director functions. And, of course, I was able to do things like calibrate the fuel senders. You can actually do quite a lot before you ever start the engine.
PS: One of the cool things was to be working in the cockpit with headphones on instead of hearing protectors and listening to the airport traffic over the intercom (as well as plugging in an MP3 player). You might say my audio system had extensive testing before the first flight; good way to get used to radio traffic again, too, if you haven't been flying while building.