Fix valgrind issue with thread_pool_test.

Make sure the task list is empty at the end of the test.

Test: thread_pool_test

Change-Id: I155a8eb90dc83a0dd9128f0c0bc64deeba7a92e9
diff --git a/runtime/thread_pool_test.cc b/runtime/thread_pool_test.cc
index 2ae2ecf..89e9005 100644
--- a/runtime/thread_pool_test.cc
+++ b/runtime/thread_pool_test.cc
@@ -114,6 +114,8 @@
   thread_pool.StopWorkers(self);
 
   thread_pool.Wait(self, false, false);  // We should not deadlock here.
+  // Drain the task list.
+  thread_pool.Wait(self, /* do_work */ true, false);  // We should not deadlock here.
 }
 
 class TreeTask : public Task {