Disable autovacuum on primary in 040_standby_failover_slots_sync test.

Disable autovacuum to avoid generating xid during stats update as
otherwise the new XID could then be replicated to standby at some random
point making slots at primary lag behind standby during slot sync.

As per buildfarm

Author: Hou Zhijie
Discussion: https://postgr.es/m/514f6f2f-6833-4539-39f1-96cd1e011f23@enterprisedb.com
Discussion: https://postgr.es/m/CAA4eK1Jun8SGCoc6JEktxY_+L7GmoJWrdsx-KCEP=GL-SsWggQ@mail.gmail.com

src/test/recovery/t/040_standby_failover_slots_sync.pl

@@ -15,6 +15,10 @@ use Test::More;
 # Create publisher
 my $publisher = PostgreSQL::Test::Cluster->new('publisher');
 $publisher->init(allows_streaming => 'logical');
+# Disable autovacuum to avoid generating xid during stats update as otherwise
+# the new XID could then be replicated to standby at some random point making
+# slots at primary lag behind standby during slot sync.
+$publisher->append_conf('postgresql.conf', 'autovacuum = off');
 $publisher->start;
 
 $publisher->safe_psql('postgres',