tests: Bluetooth: add host flow-control test

Verifies that there is no host RX buffer leak due to disconnections.

That is, not actual host buffer objects but rather the number of buffers
that the controller thinks the host has.

If there is a desynchronization between those two, the result would be that
the controller stops forwarding ACL data to the host, leading to an
eventual application timeout.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>