[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 <shixiong@databricks.com>

Closes #14884 from zsxwing/SPARK-17318.