Fix TaskMemoryManagerSuite.

Author: JoshRosen

core/src/main/java/org/apache/spark/memory/TaskMemoryManager.java

@@ -368,6 +368,7 @@ public long cleanUpAllAllocatedMemory() {
       }
       consumers.clear();
     }
+
     return memoryManager.releaseAllExecutionMemoryForTask(taskAttemptId);
   }
 

core/src/test/scala/org/apache/spark/memory/TestMemoryManager.scala

@@ -58,7 +58,9 @@ class TestMemoryManager(conf: SparkConf)
   override private[memory] def releaseExecutionMemory(
       numBytes: Long,
       taskAttemptId: Long,
-      memoryMode: MemoryMode): Unit = {}
+      memoryMode: MemoryMode): Unit = {
+    available += numBytes
+  }
   override def maxStorageMemory: Long = Long.MaxValue
 
   private var oomOnce = false