Fix query in new test to check tables are synced

Rather than looking for tablesync workers, it is more reliable to see
the sync state of the tables.

Per note from Amit Kapila.
Discussion: https://postgr.es/m/CAA4eK1JSSD7FVwq+_rOme86jUZTQFzjsNU06hQ4-LiRt1xFmSg@mail.gmail.com

src/test/subscription/t/100_bugs.pl

@@ -141,13 +141,13 @@ $node_twoways->safe_psql(
 
 # We cannot rely solely on wait_for_catchup() here; it isn't sufficient
 # when tablesync workers might still be running. So in addition to that,
-# we verify that no tablesync workers appear for the subscription.
+# verify that tables are synced.
 # XXX maybe this should be integrated in wait_for_catchup() itself.
 $node_twoways->wait_for_catchup('testsub');
-$node_twoways->poll_query_until(
-	'd2',
-	"SELECT count(*) FROM pg_stat_subscription WHERE subname = 'testsub' AND relid <> 0",
-	"0");
+my $synced_query =
+  "SELECT count(1) = 0 FROM pg_subscription_rel WHERE srsubstate NOT IN ('r', 's');";
+$node_twoways->poll_query_until('d2', $synced_query)
+  or die "Timed out while waiting for subscriber to synchronize data";
 
 is($node_twoways->safe_psql('d2', "SELECT count(f) FROM t"),
 	$rows * 2, "2x$rows rows in t");