5gc: keep NUS after UeContextRelease

Since open5gs.git b9823196b5de3394b7a144569f186d7d62fad6f9, open5gs
properly validates received seq_nr (ul_count) of integrity protected
NG-NAS message to make sure it increases with each new UL message, in
order to protect against accepting replayed messages.

This means that tests going over UeContextRelease procedure need to keep
a copy of the NUS (NAS state) because the UeContextreleaseCommand erases
the NGAP entry in the NGAP_Emulation table, together with the associated
NUS.

We need the copy of NUS because we need to keep reusing the NAS state,
increasing the previous ul_count.
This fixes tests TC_ue_service_request_cm_* as well as
TC_net_triggered_service_req.

Change-Id: I84b91bf00491a1ba0c2ee8d57e61996a88fe12d3
---
M 5gc/C5G_Tests.ttcn
M 5gc/ConnHdlr.ttcn
M library/NGAP_Emulation.ttcn
3 files changed, 81 insertions(+), 28 deletions(-)