unit-tests: Ensure listeners can track SAs via ike/child_updown/rekey()

unit-tests: Ensure listeners can track SAs via ike/child_updown/rekey()

Previously, it could happen that child_rekey() was triggered twice for
the same "old" SA.  For listeners that would mean they'd loose track as
they'd be tracking a new SA that wasn't relevant anymore and for which
no updown event would ever get triggered (it was the redundant SA in a
collision).  This new assert ensures that events are triggered in a
predictable way and listeners can track SAs properly.