[SPARK-7419] [STREAMING] [TESTS] Fix CheckpointSuite.recovery with file input stream

Fix this failure: https://amplab.cs.berkeley.edu/jenkins/job/Spark-Master-SBT/2886/AMPLAB_JENKINS_BUILD_PROFILE=hadoop2.3,label=centos/testReport/junit/org.apache.spark.streaming/CheckpointSuite/recovery_with_file_input_stream/

To reproduce this failure, you can add `Thread.sleep(2000)` before this line
https://github.com/apache/spark/blob/a9c4e29950a14e32acaac547e9a0e8879fd37fc9/streaming/src/test/scala/org/apache/spark/streaming/CheckpointSuite.scala#L477

Author: zsxwing <zsxwing@gmail.com>

Closes #7323 from zsxwing/SPARK-7419 and squashes the following commits:

b3caf58 [zsxwing] Fix CheckpointSuite.recovery with file input stream