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

(cherry-picked from commit 0608769e)