SpeakerTest: Fix subwoofer test
Move the workaround to use a different sound for the subwoofer test
to C++, so we can still use the lfe
channel and the rear-center
sound file.
Doing otherwise seems to be specially problematic on pipewire
BUG: 445523 FIXED-IN: 5.24.5
Two notes:
- I don't have a X.1 system to try this functionally
- Automatic cherry-pick to 5.24 is not possible due to a slight refactor of the code in master, but the change is trivial to port.