I don't know anything about official MLAT tests. but Flightradar24 and the other similar systems use data from large numbers of amateurs around the world.
At LTFC at Fairoaks one of the members, whose aircraft only has Mode S and not ADSB, was complaining that he can't see his aircraft on FR24 (when someone else is using it) until it's quite high up.
We've put in a 1090 receiver at the clubhouse - our simple system consists of a Raspberry Pi, a cheap DVB ADSB dongle and an £18 aerial. It contributes to MLAT calculations as well as sending position info, currently to just FR24. Sadly it turns out that you need four receivers in line of sight to do MLAT, so my colleague's aircraft will stay invisible at low level until other locals join in
Bernie