Fix timing issue in new subscription truncate test

We need to wait for the initial sync of all subscriptions.  On
some (faster?) machines, this didn't make a difference, but
the (slower?) buildfarm machines are upset.

src/test/subscription/t/010_truncate.pl

@@ -61,7 +61,11 @@ $node_subscriber->safe_psql('postgres',
 $node_subscriber->safe_psql('postgres',
 	"CREATE SUBSCRIPTION sub3 CONNECTION '$publisher_connstr application_name=sub3' PUBLICATION pub3");
 
-$node_publisher->wait_for_catchup('sub1');
+# Wait for initial sync of all subscriptions
+my $synced_query =
+"SELECT count(1) = 0 FROM pg_subscription_rel WHERE srsubstate NOT IN ('r', 's');";
+$node_subscriber->poll_query_until('postgres', $synced_query)
+  or die "Timed out while waiting for subscriber to synchronize data";
 
 # insert data to truncate