sgsn: Add test to verify duplicate DeactPDPCtx crashing osmo-sgsn

osmo-sgsn sends 2 DeletePdpCtxReq on gtp, Confirmation received on first
one frees cbp associatd to pdp and the second one accesses the already
freed cbp.

Related: OS#3956
Change-Id: Ie1cc9fdf91b18b0b7fb3d5fb33b6dcf6f46a393f
2 files changed