Changes

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
instaed, 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