tests: bsim: Bluetooth: Fail throughput test with increased Tx count

Make the throughput test to fail when increasing Tx buffers
that will delay a LE Connection Ind PDU from being
transmitted on air due to previously queued Data PDUs.

Unlike legacy Zephyr Controller's double headed Tx queue
linked list, the new split Zephyr Controller's new LLCP does
not place a control PDU at the head of the queue causing
control PDUs to be delayed on air.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>