core-test: Remove use of deprecated core/update-index.
Fix index pubsub in cores.

The config wrapper for index pubsub used the *next* core's pubsub,
instead of the current core's. That pubsub is intentionally discarded in
favor of the current core's pubsub on transition, which meant the index
was forwarding events to an orphaned pubsub registry.
Config: test index pubsub.