Use unpretty channel name in speakertest

Summary:
The speaker test did not work when channel names were translated, e.g. "Front Left" => "Vorne Links" because the comparison failed. Using pa_channel_position_to_string instead of pa_channel_position_to_pretty_string hopefully
works better.

Reviewers: drosca, broulik

Reviewed By: broulik

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14362