Stabilize subscription stats test.

The new test added by commit 68a59f9 disables the subscription and
manually drops the associated replication slot. However, since
disabling the subsubscription doesn't wait for a walsender to release
the replication slot and exit, pg_drop_replication_slot() could
fail. Avoid failure by adding a wait for the replication slot to
become inactive.

Reported-by: Hou Zhijie, as per buildfarm
Reviewed-by: Hou Zhijie
Discussion: https://postgr.es/m/OS0PR01MB571682316378379AA34854F694E9A%40OS0PR01MB5716.jpnprd01.prod.outlook.com
Backpatch-through: 15

Author: MasahikoSawada

src/test/subscription/t/026_stats.pl

@@ -285,6 +285,14 @@ sub create_sub_pub_w_errors
 		$db, qq(SELECT pg_stat_have_stats('subscription', 0, $sub2_oid))),
 	qq(f),
 	qq(Subscription stats for subscription '$sub2_name' should be removed.));
+
+# Since disabling subscription doesn't wait for walsender to release the replication
+# slot and exit, wait for the slot to become inactive.
+$node_publisher->poll_query_until(
+	$db,
+	qq(SELECT EXISTS (SELECT 1 FROM pg_replication_slots WHERE slot_name = '$sub2_name' AND active_pid IS NULL))
+) or die "slot never became inactive";
+
 $node_publisher->safe_psql($db,
 	qq(SELECT pg_drop_replication_slot('$sub2_name')));