Fix regression test failure when wal_level is set to minimal.

Fix regression test failure when wal_level is set to minimal.

Commit 67c209 removed the WARNING for insufficient wal_level from the
expected output, but the WARNING may still appear on buildfarm members
that run with wal_level=minimal.

To avoid unstable test output depending on wal_level, this commit the
test to use ALTER PUBLICATION for verifying the same behavior,
ensuring the output remains consistent regardless of the wal_level
setting.

Per buildfarm member thorntail.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Discussion: https://postgr.es/m/TY4PR01MB16907680E27BAB146C8EB1A4294B2A@TY4PR01MB16907.jpnprd01.prod.outlook.com