How do professional companies test a low power wireless network performance?
I've seen a few wireless testbeds for 2.4 GHz (802.15.4) networks, including multi-hop links, deployed in both academic and industrial (though R&amp;D) environments. None of these was for certification of link performance, but more to generally assess the feasibility of the given architecture with the to-be-tested hardware and protocol.

The networks were deployed trying to replicate a typical 'sound' installation therefore with a backbone running through central spaces (like corridors) and with line of sight between the nodes, and several other nodes scattered around in the rooms.

If you want to test the maximum reach of the nodes, then you're better served by going to an open space and putting the nodes within line of sight. Even better if you can raise them as much as possible over the ground, which may be a source of reflection and/or attenuation. If instead you want to experiment real-world performance in the indoor setting you describe, you can try the above approach, plus you can try using two (or more) nodes in two separate rooms with different degrees of separation, like drywall, stone wall, glass and whatever you can come up with.

Also note that with higher-level protocols, you may have some trouble checking for errors, as the protocol will try to auto-correct and you will just be able to measure the effective speed or a calculated metric of signal strength.