misc: Address follow up comment re. abort exchange test (#11966)

Summary:
Pull Request resolved: #11966

Assert on the use_count condition of the merge task instead of waiting for sleep to reduce flakiness

Reviewed By: xiaoxmeng

Differential Revision: D67655127

fbshipit-source-id: bf4dd2ba5027c990fd0edcf868ec3a2d9b44458a

velox/exec/tests/MultiFragmentTest.cpp

@@ -512,9 +512,11 @@ TEST_P(MultiFragmentTest, abortMergeExchange) {
   // Ensure that the threads in the executor can gracefully join
   executor.join();
 
-  /* sleep override */
-  // Wait till all the terminations, closures and destructions are done.
-  std::this_thread::sleep_for(std::chrono::seconds(1));
+  // Wait till all the terminations, closures and destructions so that
+  // the reference count drops to 2.
+  while (mergeTask.use_count() > 2) {
+    std::this_thread::yield();
+  }
 
   // The references to mergeTask should be two, one for the local variable
   // itself and one reference inside tasks variable.