s1gw: add tests for MME registry REST procedures

Add three test cases exercising the S1GW REST interface for MME pool
management.  The REST TCs are gated on the mp_rest_enable module
parameter in the control block.

TC_rest_mme_list: query the MME pool list via REST and verify it
matches the three static entries from the 'mme_pool' section in
osmo-s1gw.config (mme0/mme1/mme2 with their respective addresses).

TC_rest_mme_add_del: add a new MME entry at runtime via REST, verify
it appears in both the list and individual GET responses, then delete
it and confirm it is gone.

TC_rest_mme_del_fallback: delete mme0 from the pool at runtime and
verify that a connecting eNB is routed directly to mme1, skipping the
deleted entry.  The pool is restored to its original state afterwards
via f_REST_mme_pool_restore().

Also add:
* {ts,tr}_MmeItem templates to S1GW_REST_Types.ttcn
* f_REST_mme_find(): returns the integer index of a named entry in a
  MmeList, or -1 if not found; used for both presence and absence checks
* f_REST_mme_pool_restore(): deletes all current entries and re-adds
  mme0/mme1/mme2 in original order to keep pool state predictable
  across test cases

Change-Id: I260bc987ab8ae0ecb547d0b69b261fd97c5c9c23
Related: SYS#7052
---
M s1gw/S1GW_REST_Types.ttcn
M s1gw/S1GW_Tests.ttcn
M s1gw/expected-results.xml
3 files changed, 199 insertions(+), 1 deletion(-)