Fix flaky citus_non_blocking_split_shard_cleanup (#7311)

Sometimes in CI citus_non_blocking_split_shard_cleanup failed like this:

```diff
--- /__w/citus/citus/src/test/regress/expected/citus_non_blocking_split_shard_cleanup.out.modified	2023-11-01 15:07:14.280551207 +0000
+++ /__w/citus/citus/src/test/regress/results/citus_non_blocking_split_shard_cleanup.out.modified	2023-11-01 15:07:14.292551358 +0000
@@ -106,21 +106,22 @@
 -----------------------------------

 (1 row)

 \c - - - :worker_2_port
 SET search_path TO "citus_split_test_schema";
 -- Replication slots should be cleaned up
 SELECT slot_name FROM pg_replication_slots;
             slot_name
 ---------------------------------
-(0 rows)
+ citus_shard_split_slot_19_10_17
+(1 row)

 -- Publications should be cleanedup
 SELECT count(*) FROM pg_publication;
  count
```

It's expected that the replication slot is sometimes not cleaned up if
we don't wait until resource cleanup completes. This PR starts doing
that here.

src/test/regress/expected/citus_non_blocking_split_shard_cleanup.out

@@ -107,6 +107,12 @@ SELECT pg_catalog.citus_split_shard_by_split_points(
 
 (1 row)
 
+SELECT public.wait_for_resource_cleanup();
+ wait_for_resource_cleanup
+---------------------------------------------------------------------
+
+(1 row)
+
 \c - - - :worker_2_port
 SET search_path TO "citus_split_test_schema";
 -- Replication slots should be cleaned up

src/test/regress/sql/citus_non_blocking_split_shard_cleanup.sql

@@ -79,6 +79,8 @@ SELECT pg_catalog.citus_split_shard_by_split_points(
     ARRAY[:worker_2_node, :worker_2_node, :worker_2_node],
     'force_logical');
 
+SELECT public.wait_for_resource_cleanup();
+
 \c - - - :worker_2_port
 SET search_path TO "citus_split_test_schema";
 -- Replication slots should be cleaned up