[SPARK-4790][STREAMING] Fix ReceivedBlockTrackerSuite waits for old file...

...s to get deleted before continuing.

Since the deletes are happening asynchronously, the getFileStatus call might throw an exception in older HDFS
versions, if the delete happens between the time listFiles is called on the directory and getFileStatus is called
on the file in the getFileStatus method.

This PR addresses this by adding an option to delete the files synchronously and then waiting for the deletion to
complete before proceeding.
I think its better to not expose implementation details like asynchronous-ness in the signature (i.e. returning `Future`) unless absolutely necessary. 

Also, the timeout is only for testing, so this is not for actual usage. So its okay to have 1 second.