s1gw: add testcases for impatient eNB during MME pool selection
Two new test cases covering scenarios where the eNB disconnects before S1 setup completes, targeting specific states of the enb_proxy FSM:
* TC_mme_pool_enb_disc_wait_s1setup_req: eNB connects but disconnects before sending S1SetupReq (enb_proxy in wait_s1setup_req). No MME connection is ever attempted; S1GW must handle the disconnect cleanly.
* TC_mme_pool_enb_disc_wait_s1setup_rsp: eNB sends S1SetupReq, S1GW forwards it to the first pool MME (enb_proxy in wait_s1setup_rsp), then eNB disconnects before the response arrives. S1GW must detect the eNB disconnect and close the open MME connection in response.
A new helper S1GW_ConnHdlr.f_ConnHdlr_s1ap_close() is added for these tests: unlike f_ConnHdlr_s1ap_disconnect(), it closes the eNB-side socket without waiting for an S1APSRV_EVENT_CONN_DOWN from a pool server (since in these scenarios either no MME connection exists yet, or the CONN_DOWN is captured by the test body directly).
