tests: Bluetooth: df: Add test to verify correctness of relase of PDUs

Add unit tests that will ensure the CTE disable operation does not
cause breaking of LLL operations by too early release of chained PDUs.
The tests verify if numbers of PDUs in free PDUs fifo and free PDUs
memory pool are correct.

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