Fix ThreadPool events validation test (#48588)

* Fix ThreadPool events validation test

* space

src/tests/tracing/eventpipe/eventsvalidation/ThreadPool.cs

@@ -48,6 +48,7 @@ public static int Main(string[] args)
                     return total;
                 });
             }
+            Task.WaitAll(tasks);
         };
 
         private static Func<EventPipeEventSource, Func<int>> _DoesTraceContainEvents = (source) =>