[SPARK-17318][TESTS] Fix ReplSuite replicating blocks of object with class defined in repl

## What changes were proposed in this pull request?

There are a lot of failures recently: http://spark-tests.appspot.com/tests/org.apache.spark.repl.ReplSuite/replicating%20blocks%20of%20object%20with%20class%20defined%20in%20repl

This PR just changed the persist level to `MEMORY_AND_DISK_2` to avoid blocks being evicted from memory.

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <[email protected]>

Closes #14884 from zsxwing/SPARK-17318.

(cherry picked from commit 231f973)
Signed-off-by: Shixiong Zhu <[email protected]>

Author: zsxwing

repl/scala-2.11/src/test/scala/org/apache/spark/repl/ReplSuite.scala

@@ -401,7 +401,7 @@ class ReplSuite extends SparkFunSuite {
       """
         |import org.apache.spark.storage.StorageLevel._
         |case class Foo(i: Int)
-        |val ret = sc.parallelize((1 to 100).map(Foo), 10).persist(MEMORY_ONLY_2)
+        |val ret = sc.parallelize((1 to 100).map(Foo), 10).persist(MEMORY_AND_DISK_2)
         |ret.count()
         |sc.getExecutorStorageStatus.map(s => s.rddBlocksById(ret.id).size).sum
       """.stripMargin)