tests: Bluetooth: Audio: Open backchannel to self

In order to use the device IDs send sending to a specific device,
e.g. backchannel_sync_send(3), then we need to open all backchannels
including the one to ourself, otherwise the ID and count will be
off by one.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>