Don't log error when dropping non-existent replication slot in tests

In io.debezium.connector.postgresql.TestHelper.dropDefaultReplicationSlot() we unconditionally try to drop the default replication slot. Often times, it will not exist as it has been dropped by the previous test already. Let's suppress the logged exception in this case, it only clutters the log. Exclusion should be for the specific message, so to not hide other, unrelated exceptions.