Fix valgrind issue with thread_pool_test.

Make sure the task list is empty at the end of the test. Follow-up
to commit e2fc98e88106b9cfa44eeacc6765c291feda04a8.

Test: m test-art-host-gtest-thread_pool_test
Test: m valgrind-test-art-host-gtest-thread_pool_test
Change-Id: Ib41a1f7dde9d45f1195e18ff6db4241fb7dc7428
diff --git a/runtime/thread_pool_test.cc b/runtime/thread_pool_test.cc
index 89e9005..14c2c3b 100644
--- a/runtime/thread_pool_test.cc
+++ b/runtime/thread_pool_test.cc
@@ -114,8 +114,11 @@
   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.
+
+  // Drain the task list. Note: we have to restart here, as no tasks will be finished when
+  // the pool is stopped.
+  thread_pool.StartWorkers(self);
+  thread_pool.Wait(self, /* do_work */ true, false);
 }
 
 class TreeTask : public Task {