Added unit tests for ClusterTaskSetManager, and fix a bug found with

core/src/main/scala/spark/scheduler/TaskLocation.scala

core/src/main/scala/spark/scheduler/TaskResult.scala

...scala/spark/scheduler/cluster/ClusterTaskSetManager.scala

.../main/scala/spark/scheduler/cluster/TaskDescription.scala

...c/main/scala/spark/scheduler/cluster/TaskSetManager.scala

...ain/scala/spark/scheduler/local/LocalTaskSetManager.scala

core/src/main/scala/spark/util/Clock.scala

...scala/spark/scheduler/cluster/ClusterSchedulerSuite.scala

.../spark/scheduler/cluster/ClusterTaskSetManagerSuite.scala

core/src/test/scala/spark/scheduler/cluster/FakeTask.scala

core/src/test/scala/spark/util/FakeClock.scala