Fixes flakiness in background_rebalance_parallel test (#6910)

Fixes the following flaky outputs by decreasing citus_task_wait loop
interval, and changing the order of wait commands.

https://app.circleci.com/pipelines/github/citusdata/citus/32102/workflows/19958297-6c7e-49ef-9bc2-8efe8aacb96f/jobs/1089589

``` diff
SELECT job_id, task_id, status, nodes_involved
 FROM pg_dist_background_task WHERE job_id in (:job_id) ORDER BY task_id;
  job_id | task_id |  status  | nodes_involved 
 --------+---------+----------+----------------
   17779 |    1013 | done     | {50,56}
   17779 |    1014 | running  | {50,57}
-  17779 |    1015 | running  | {50,56}
-  17779 |    1016 | blocked  | {50,57}
+  17779 |    1015 | done     | {50,56}
+  17779 |    1016 | running  | {50,57}
   17779 |    1017 | runnable | {50,56}
   17779 |    1018 | blocked  | {50,57}
   17779 |    1019 | runnable | {50,56}
   17779 |    1020 | blocked  | {50,57}
 (8 rows)
```

https://github.com/citusdata/citus/pull/6893#issuecomment-1525661408
```diff
SELECT job_id, task_id, status, nodes_involved
 FROM pg_dist_background_task WHERE job_id in (:job_id) ORDER BY task_id;
  job_id | task_id |  status  | nodes_involved 
 --------+---------+----------+----------------
   17779 |    1013 | done     | {50,56}
-  17779 |    1014 | running  | {50,57}
+  17779 |    1014 | runnable | {50,57}
   17779 |    1015 | running  | {50,56}
   17779 |    1016 | blocked  | {50,57}
   17779 |    1017 | runnable | {50,56}
   17779 |    1018 | blocked  | {50,57}
   17779 |    1019 | runnable | {50,56}
   17779 |    1020 | blocked  | {50,57}
 (8 rows)
```

src/backend/distributed/utils/background_jobs.c

@@ -395,7 +395,7 @@ citus_task_wait_internal(int64 taskid, BackgroundTaskStatus *desiredStatus)
 
 		/* sleep for a while, before rechecking the task status */
 		CHECK_FOR_INTERRUPTS();
-		const long delay_ms = 1000;
+		const long delay_ms = 100;
 		(void) WaitLatch(MyLatch,
 						 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
 						 delay_ms,

src/test/regress/expected/background_rebalance_parallel.out

@@ -513,6 +513,12 @@ FROM pg_dist_background_task WHERE job_id in (:job_id) ORDER BY task_id;
 (8 rows)
 
 -- increase citus.max_background_task_executors_per_node
+SELECT citus_task_wait(1013, desired_status => 'done');
+ citus_task_wait
+---------------------------------------------------------------------
+
+(1 row)
+
 ALTER SYSTEM SET citus.max_background_task_executors_per_node = 2;
 SELECT pg_reload_conf();
  pg_reload_conf
@@ -520,13 +526,13 @@ SELECT pg_reload_conf();
  t
 (1 row)
 
-SELECT citus_task_wait(1015, desired_status => 'running');
+SELECT citus_task_wait(1014, desired_status => 'running');
  citus_task_wait
 ---------------------------------------------------------------------
 
 (1 row)
 
-SELECT citus_task_wait(1013, desired_status => 'done');
+SELECT citus_task_wait(1015, desired_status => 'running');
  citus_task_wait
 ---------------------------------------------------------------------
 

src/test/regress/sql/background_rebalance_parallel.sql

@@ -221,10 +221,12 @@ SELECT job_id, task_id, status, nodes_involved
 FROM pg_dist_background_task WHERE job_id in (:job_id) ORDER BY task_id;
 
 -- increase citus.max_background_task_executors_per_node
+SELECT citus_task_wait(1013, desired_status => 'done');
 ALTER SYSTEM SET citus.max_background_task_executors_per_node = 2;
 SELECT pg_reload_conf();
+
+SELECT citus_task_wait(1014, desired_status => 'running');
 SELECT citus_task_wait(1015, desired_status => 'running');
-SELECT citus_task_wait(1013, desired_status => 'done');
 
 -- show that at most 2 tasks per node are running
 -- among the tasks that are not blocked