Add missing FATAL => 'all' to a use warnings in Perl

Author: Anton Voloshin <a.voloshin@postgrespro.ru>
Discussion: https://www.postgresql.org/message-id/aa8a55d5-554a-4027-a491-1b0ca7c85f7a@postgrespro.ru

src/test/modules/test_misc/t/006_signal_autovacuum.pl

@@ -7,7 +7,7 @@
 # at the beginning of the autovacuum worker startup.
 
 use strict;
-use warnings;
+use warnings FATAL => 'all';
 use PostgreSQL::Test::Cluster;
 use Test::More;