The two test stands that SpaceX has in McGregor, TX are pretty open at the base.
There is the BFT (Big F*****/Falcon) Test stand that they inherited and upgraded.
It has since been retired (at least for now... My personal theory is that as the launch tempo increases they will possibly require reactivating it) since the change to super cooled LOX and RP1 would require a lot of ground support equipment (GSE) changes.
They were in the midst of building a new test stand, we all thought was for the Falcon Heavy, so needed to be big enough to handle three cores at once, though since then it seems they only plan on testing one core at a time, however as the Falcon Heavy launch campaign is getting closer and closer, reality will trump theory.
You can see that test stand in this photo:
What you will notice in both is that they are REALLY open to the world. There is a lot of space for the exhaust to go, and nowhere really to pour water that can absorb the energy (thermal and audible). Apparently they do use a water deluge system, and it is likely at the bottom of the new test stands trench.