Improve the stability of the recovery test 047_checkpoint_physical_slot

Improve the stability of the recovery test 047_checkpoint_physical_slot

Currently, the comments in 047_checkpoint_physical_slot. It shows an
incomplete intention to wait for checkpoint completion before performing
an immediate database stop.  However, an immediate node stop can occur both
before and after checkpoint completion.  Both cases should work correctly.
But we would like the test to be more stable and deterministic.  This is why
this commit makes this test explicitly wait for the checkpoint completion
log message.

Discussion: https://postgr.es/m/CAPpHfdurV-j_e0pb%3DUFENAy3tyzxfF%2ByHveNDNQk2gM82WBU5A%40mail.gmail.com
Discussion: https://postgr.es/m/aHXLep3OaX_vRTNQ%40paquier.xyz
Author: Alexander Korotkov <akorotkov@postgresql.org>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Backpatch-through: 17