After scouring the internet, the device seen in the first two pictures is indeed a clock.
In the linked video the display does count up like a clock. In the beginning of the video the narrator says, "it's just after 4 AM in Seattle," and the displayed time is 04:17:58.
Watching the maiden flight of the A350, no similar device is found.
I then turned my attention to the FAA's Flight Test Guide For Certification Of Transport Category Airplanes, there's no mention of such a device being mandatory.
Closest thing I found is a requirement to video record and timestamp spin tests in light aircraft.
The timestamping theory would make sense if the video recording of the cockpit is high speed, which would be overkill for any scenario really.
Time synchronization for the flight test instrumentation data acquisition is a real, and complicated thing.
My theory, showing the pilots the master clock—if that's the case—would help the pilots note the time when a situation arises so the engineers can check the relevant data.
It might also have other functions as recording total elapsed time of the entire flight test program for the airframe it's installed on.
I've also looked into trajectory guidance systems for test pilots based on true airspeed/pressure measurements being fed to the pilots, but such systems turned out to look completely different than a seven-segment display.
Over to you.