Changes

pcu: Fix TC_dl_cs1_to_cs4 sporadic fail

The test TC_dl_cs1_to_cs4 failed sporadically in ttcn3-pcu-test-asan.
Due to how the DL data arriving at Gb is split in chunks over RLC/MAC
(also based on how CS changes over time), it may happen that the full
PDU content doesn't finish at the exact block number where PCU expects
the DL ACK/NACK. As a result, since PCU delays finishing the DL TBF and
some data for that DL TBF has already not been ACKed (and since there's
no more active DL TBFs), it will decide to retransmit some of the
RLC/MAC blocks which haven't been yet ACKed, instead of transmitting
nothing.
This is an optimization to increase the probabilities the MS has
received all the data.
We need to account for this possibility in f_dl_data_exp_cs(), used in
the mentioned test. In there, it needs to be checked whether the
received DL data block is a retransmission, and use that knowledge to
resolve that all data has been transmitted and hence the final condition
can be checked.

Change-Id: I7fb40689547645adf1d54f43c263161be1d7a440