Fix a test in 003_logical_slots.

Fix a test in 003_logical_slots.

We allow to upgrade the slot iff there is no pending WAL to be processed.
The test first disables the subscription to avoid unnecessary LOGs on the
subscriber and then stops the publisher node. It is quite possible that
just before the shutdown of the publisher, autovacuum generates some WAL
record that needs to be processed, so just disable the autovacuum for this
test.

Per buildfarm.

Author: Hayato Kuroda
Reviewed-by: Amit Kapila
Discussion: http://postgr.es/m/OS3PR01MB9882FED1F0060468FB01B9DAF583A@OS3PR01MB9882.jpnprd01.prod.outlook.com