[SPARK-17778][TESTS] Mock SparkContext to reduce memory usage of BlockManagerSuite

## What changes were proposed in this pull request?

Mock SparkContext to reduce memory usage of BlockManagerSuite

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15350 from zsxwing/SPARK-17778.

core/src/test/scala/org/apache/spark/storage/BlockManagerSuite.scala

@@ -107,7 +107,10 @@ class BlockManagerSuite extends SparkFunSuite with Matchers with BeforeAndAfterE
     rpcEnv = RpcEnv.create("test", "localhost", 0, conf, securityMgr)
     conf.set("spark.driver.port", rpcEnv.address.port.toString)
 
-    sc = new SparkContext("local", "test", conf)
+    // Mock SparkContext to reduce the memory usage of tests. It's fine since the only reason we
+    // need to create a SparkContext is to initialize LiveListenerBus.
+    sc = mock(classOf[SparkContext])
+    when(sc.conf).thenReturn(conf)
     master = new BlockManagerMaster(rpcEnv.setupEndpoint("blockmanager",
       new BlockManagerMasterEndpoint(rpcEnv, true, conf,
         new LiveListenerBus(sc))), conf, true)