tests: Bluetooth: don't assume handle is 0

Don't assume the connection handle will always be 0. This is currently the
case with the Zephyr link layer, but recently changed in the Softdevice
Controller.

Another problem is that apparently the spec allows the controller to just
blackhole data instead of returning `BT_HCI_ERR_UNKNOWN_CONN_ID` if the
host uses a wrong handle.

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