tests: Bluetooth: df: Add tests for set conn CTE TX params

Add unit tests for HCI command HCI_LE_Set_Connection_CTE_-
Transmit_Parameters.

Part of a code that was available in df/connection_cte_req/common.c,
related with artificial handling of connection, is moved to
df/common/bt_conn_common.c.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>