tests: Bluetooth: Unicast client BSIM used group size as stream_cnt

Instead of setting the stream_cnt based on the number
of streams configurer, we use the number of streams actually
added to the group, and then configure that amount.

This should ensure that we do not try to configure any
streams that we have not already added to the group.

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