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
1 file changed