SGSN: write regression tests for Authentication
We need end-to-end tests for the several types of authentication in the SGSN.
Test all of these:
- No authentication
- GSM AKA
- UMTS AKA without AUTS sync
- UMTS AKA with AUTS sync
- UMTS AKA in pre-R99 compatibility mode
Having in mind something like the msc_vlr tests where messages are fabricated
from the MS and the HLR sides from hex strings to play through the various cases.