Changes

s1gw: f_ConnHdlr_session_delete(): invalidate peer's SEID

Change-Id: If8027e2bdb5822eb7555cd02a72be900a1f18880

s1gw: f_ConnHdlr_session_delete(): respect any order

It's not guranteed (nor required) that PFCP Session Deletion Request
PDUs are sent in the same order as their respective ERab records are
organized in the given ERabList.  They can be emitted in any order.

Make f_ConnHdlr_session_delete() more flexible:

* Expect to receive N PFCP Session Deletion Request PDUs;
* For each received PFCP PDU, find the matching E-RAB;
* Make sure that an E-RAB is never released twice;
* Send PFCP Session Deletion Response.

This patch fixes the following TCs:

* TC_ue_ctx_release_req3
* TC_ue_ctx_release_cmd_compl3

Change-Id: I93d3d38829b1f425d18d59464894da239e3511eb
Related: 75cb01b7 ("s1gw: add TCs for UE CONTEXT RELEASE procedures")
Related: SYS#7310

s1gw: add TC_e_rab_modify_req_rsp[_multi]

Test the E-RAB Modify procedure (MME initiated), which is defined
in 3GPP TS 36.413 section 8.2.2.

Change-Id: I58b9964bae24b5aac526ee0c0c8ac8798f351e05
Related: osmo-s1gw.git I24d57ea390d71b640cb42046a8658e9d30670682
Related: osmo-s1gw.git I057a1c58d0c6ad04c00ad3ea4c68230d8632a07e
Related: SYS#7308

s1gw: add TC_e_rab_modify_ind_cnf[_multi]

Test the E-RAB Modify procedure (eNB initiated), which is defined
in 3GPP TS 36.413 section 8.2.4.

Change-Id: Iec95ca0ecf37290bddfbefce1861f8ae66bf4db1
Related: osmo-s1gw.git I750ada0a5a21edc8bc06d567c8000b6304966474
Related: osmo-s1gw.git Icc16a57d40b68bd17c5d207b643927d58176e088
Related: SYS#7308