Changes

s1ap_proxy: correct handling of UE CONTEXT RELEASE REQUEST

By sending a UE CONTEXT RELEASE REQUEST, the eNB is requesting the
MME to terminate the UE context.  This procedure itself does not
terminate the UE context nor the associated E-RABs.  Therefore, the
S1GW should not terminate E-RAB FSMs upon receiving this PDU.

Keep the function clause in s1ap_proxy, so that the respective
?S1GW_CTR_S1AP_PROXY_IN_PKT_RELEASE_CTX_REQ is properly updated.
Correct the unit test: E-RAB is now expected to remain alive.

Change-Id: If467162e871b7989f0578be30c04d1c32c263c99
Related: osmo-ttcn3-hacks.git Ibbb6da1f2888995608b980e7479758000e94864e
Related: SYS#7738, SYS#7599

s1ap_proxy_test: update TC scenario for test_drop_proc_error

A follow-up commit implements handling of late E-RAB setup response.
This testcase is going to fail because, by coincidence, currently
we simulate the scenario of late INITIAL CONTEXT SETUP RESPONSE and
expect the s1ap_proxy to drop the PDU.  Change the testcase scenario
to simulate an INITIAL CONTEXT SETUP RESPONSE without prior REQUEST
instead, so that it won't be affected by the upcoming patch.
Take a chance to add asserts for per-eNB metrics.

Change-Id: I0b24807c9cf08564b53b9d0e087b797fea66e565
Related: SYS#7738, SYS#7599

s1ap_proxy_test: add TCs simulating late E-RAB SETUP Rsp

It happens quite often in production that a UE context is being
established and then released even before the eNB acknowledges its
establishment.  Add a unit tests for this scenario, preparing for
a follow-up patch making the E-RAB FSM more flexible.  The PDUs
triggering late E-RAB SETUP Rsp event are currently being dropped
because they reference no longer existing E-RABs.

Change-Id: I60882c728ff06fb71cc458e807a49ca8ba5adcff
Related: SYS#7738, SYS#7599

erab_fsm: handle E-RAB SETUP Rsp during release

It is quite common in production for a UE context to be established
and then released before the eNB has acknowledged its creation.  In
the current implementation, this leads to errors and causes the PDU
that triggers the late E-RAB SETUP Response event to be dropped.

Make the FSM logic more flexible by handling late erab_setup_rsp
event in state erab_wait_release_rsp, i.e. during the release.
This way we avoid dropping PDUs and printing errors.

Change-Id: I17414aad42507b64e933c80a220bfbe3798a4ad0
Related: SYS#7738, SYS#7599