We've hit on a system during testing that seems to have a broken ACPI interface for the system backlight. I believe it's important this interface works, since it will be used by gnome-settings-daemon where it exists - due to logic in that code that dictates where multiple backlight interfaces are available, the one of type 'firmware' (as shown in /sys/class/backlight/<backlight>/type) is chosen. This is typically 'acpi_video0'. On the system in question, using the slider in gnome-control-center will update '/sys/class/backlight/acpi_video0/brightness', but this is not reflected in 'actual_brightness' and more importantly not reflected in the backlight hardware.
I think there should be a new test which checks that, if the system has a firmware interface to the backlight then updating 'brightness' will reflect in 'actual_brightness'.
We have a fwts "brightness" test, but this is an interactive test because it also exercises the brightness control keys as well inspecting the brightness levels settings. It's broken, but I've just got a fix to it (LP: #1250517).
I guess you want this as a non-interactive test?