This change refactors the rlc_am_nr_status_pdu_t from a simple
container into a class with integrated counter of the packed size.
As this depends on the SN length, the SN length must be passed
via constructor.
* made some methods const
* changed from TESTASSERT to TESTASSERT_EQ
* fix up comment
* removed superfulous if
* used typedef for list of segments received
* added unit test for is_retx_required()
* added unit test for malformed status PDU