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Years ago I was purchasing some MV parts from an old time dealer/wrecker/tow truck operator. He moved things around his extremely cluttered yard by means of two once mobile cranes, one toward each end of the yard. We were standing alongside one when he muttered something about seeing if it would go. Picking up a battery cable with nothing but a fuzzy end on it he dobbed it on the terminal of a nearby battery and the 4 cylinder Dorman diesel that powered the crane awoke from its slumber and settled into a steady idle. Looking at the engine in disbelief I noticed it wasn't connected to the radiator in any way. I pointed this out to the old gentleman and with a dismissive gesture he said, 'it'll run all day like that, been that way for years'.
I've played around with a few engines in my time and don't hesitate to run them for a short period with out coolant in the system. I don't believe I've done them any harm.
In regards to testing a rebuilt engine before installation, I would certainly run it for a longer period which does necessitate the use of coolant. I knew one backyard rebuilder who would hook a garden hose up to the engine he was running with a drain tube on the outlet to direct the water down an open gutter. Laying in the gutter was a thermometer. He adjusted the water flow to get the desired temperature.
I rebuilt a Dodge T245 6 cylinder side valve engine on one occasion and when I set it up on the stand and coupled up a radiator for an extended run there was water trickling out the side of the crank case. When we looked closely there was an 8 inch crack along the bottom of the water jacket. I hadn't crack tested the block which was a real oversight but it would have been worse if I had simply put the engine in the truck before giving it a proper run.

David