Stabilize subscription stats test.

Stabilize subscription stats test.

The new test added by commit 68a59f9e9 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