Improve recovery test 046_checkpoint_logical_slot

Improve recovery test 046_checkpoint_logical_slot

This commit improves 046_checkpoint_logical_slot in two aspects:
- Add one pg_logical_emit_message() call to force the creation of a record
  that spawns across two pages.
- Make the logic wait for the checkpoint completion.

Discussion: https://postgr.es/m/CALDaNm34m36PDHzsU_GdcNXU0gLTfFY5rzh9GSQv%3Dw6B%2BQVNRQ%40mail.gmail.com
Author: Alexander Korotkov <akorotkov@postgresql.org>
Co-authored-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>