If you look on Google Maps, at the McGregor, TX test site, you can see the old test stand (Labelled A) where they test each Falcon 9 first stage for a 9 engine fire as an acceptance test before sending on to the launch facility.
You can see a nice view of a stage on that vertical test stand in this image:
Labelled B in my first image is the new horizontal test stand. You can see the very large 'trough' or exhaust vent they built. Initial speculation was that they would test Falcon Heavy for all in testing.
But does it make any sense to test each Falcon Heavy, all three cores, at once, before launch? For Development, it makes sense. For every launch, does it? Test the individual core, and then assemble at the pad.
Additional speculation suggests that since this is scaled for a 3+ million lb thrust vehicle, is that it would be good for Raptor engine testing, which floats around the 1 to 1.5 million pound thrust range. (Seems to change regularly. Development program I guess).
Still probably too small for a full up BFR test, which is likely to have an odd number of engines (3, 5, 7, 9, 13) to allow for a center engine for landing. (Where does one test a 13 engine, 1.5 million lb thrust vehicle? (almost 20 million lbs of thrust)).