Wait for task completion future in cursor (facebookincubator#9225)

Summary:
Pull Request resolved: facebookincubator#9225

Wait for the taskCompletionFuture in MultiTaskCursor's moveNext() method, currently we just fetch it.

(This was my bad, I thought it waited like waitForTaskDriversToFinish, but it just returns the future as the name suggests)

Reviewed By: xiaoxmeng

Differential Revision: D55261345

fbshipit-source-id: 540781ddec6608f1717991cb29f44c538f160439

velox/exec/tests/utils/Cursor.cpp

@@ -277,7 +277,7 @@ class MultiThreadedTaskCursor : public TaskCursorBase {
     if (task_->error()) {
       // Wait for the task to finish (there's' a small period of time between
       // when the error is set on the Task and terminate is called).
-      task_->taskCompletionFuture(1'000'000);
+      task_->taskCompletionFuture(1'000'000).wait();
 
       // Wait for all task drivers to finish to avoid destroying the executor_
       // before task_ finished using it and causing a crash.