Reintroduce test 046_checkpoint_logical_slot

Reintroduce test 046_checkpoint_logical_slot

This commit is only for HEAD and v18, where the test has been removed.
It also incorporates improvements below to stability and coverage of the
original test, which were already backpatched to v17.
- 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.

Author: Alexander Korotkov <akorotkov@postgresql.org>
Co-authored-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Backpatch-through: 18