There are places that can perform tests on the ABS block controller including connecting it to loads, but that would mean taking the car off the road for a few days probably.
I can sympathise with you, back in the darker days when these Bosch ABS controller had not much in the way of self checking, I had a slightly used 1991 VX Cav GSI 2000 16V 4X4, and while under used car warranty it switched on its ABS light, I booked it in, "front near side sensor faulty", paid my part of the repair supposedly cheaper than "doing it the other way"??? - this was the VX main dealer and seller of this car. Drove 4 yards, ABS light back on, turned round and demanded they sort it next day, handed it it next day, "replaced front off side sensor", same thing, the next day "replaced both rear sensors" - I asked if they could diagnose this system and they absolutely, and I asked if they had been road testing after repair again "absolutely". The weekend arrived and I unplugged the ABS controller and carried out impedance testing both down the cable and into the ABS controller and also spun each wheel in turn and measured the low voltage output from each sensor, everything pointed to a failed ABS controller ie a failed single channel, so back I went with this info only to be told that they had the diagnostic tools and were fit to use them, the car came back that evening "replaced both front sensors, road tested by workshop foreman and passed", same thing after roughly 400 yards, a bit more than just annoying as this was happening in deepest winter where ABS is quite handy to have! After contacting the service manager, I got a phone call from the service desk to say that they were ordering in a new ABS controller and and would contact me when it arrived, a few days later they did make contact, I drove round to the workshop, and waited as I knew where the controller was located, under the passenger side door sill trim, and that was that all sorted after a total of 6 wheel sensor replacements then the expensive ABS controller, I was only ever charged for the single ABS sensor replacement also luckily I worked only a 100 or so yards from that VX dealership!
So, 2 years later, the ABS light came on again, this time as it was out of warranty, I handed it in to a proper Bosch workshop, they reported that VX would not have been able to extract any fault codes from that version of Bosch ABS controller, so had just been guessing, also, each time they had removed a sensor they had binned it and its PTFE sleeve so every ABS sensor fitted after that without a PTFE sleeve was a time bomb as its aluminium sleeve/body reacted with the cast steel hub housings and the salts generated forced the ABS sensor casing to implode sort off and short out the windings - nice!
I had reason to re-visit that dealership after it moved sites and ended up chatting to the same service manager who assured me that nothing like that could ever have happened, it must be nice to live in a magic world where you can only do good things.