Fix logical decoding test to correctly check slot removal on standby.

Fix logical decoding test to correctly check slot removal on standby.

The regression test for logical decoding verifies whether a logical slot
is correctly dropped on a standby when its associated database is dropped.
However, the test mistakenly retrieved slot information from the primary
instead of the standby, causing incorrect behavior.

This commit fixes the issue by ensuring the test correctly checks the slot
on the standby.

Back-patch to all supported versions.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: http://postgr.es/m/1fdfd020-a509-403c-bd8f-a04664aba148@oss.nttdata.com
Backpatch-through: 13