[#1254] fix(test): Fix the flaky test RssShuffleTest. (#1255)

### What changes were proposed in this pull request?
As title.

### Why are the changes needed?

Fix: #1254 

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

unit test

client-tez/src/test/java/org/apache/tez/runtime/library/common/shuffle/orderedgrouped/RssShuffleTest.java

@@ -122,9 +122,9 @@ public void testKillSelf() throws IOException, InterruptedException {
       conf.setLong(Constants.TEZ_RUNTIME_TASK_MEMORY, 300000L);
       RssShuffle shuffle = new RssShuffle(inputContext, conf, 1, 3000000L, 0, APPATTEMPT_ID);
       try {
-        shuffle.run();
         ShuffleScheduler scheduler = shuffle.rssScheduler;
         assertFalse(scheduler.isShutdown());
+        shuffle.run();
 
         // killSelf() would invoke close(). Internally Shuffle --> merge.close() --> finalMerge()
         // gets called. In MergeManager::finalMerge(), it would throw illegal argument exception