tests: Bluetooth: Tester: Add missing checks for tx in bcast

The stream_started and stream_stopped callbacks are called for
both broadcast sink and source in the BT Tester, and thus
they should check if the stream can send before attempting
to register it for TX (which would otherwise fail).

This also guards the call to btp_send_bis_syced_ev so that
the event is only sent for broadcast sinks.

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