Adding unit test for FileNotFoundException when split a wal in SplitLogWorker

In HBase-20583, it is proposed a buggy situation in which an FileException would not be handled in SplitLogWorker. However, in HBase-20583, the test patch is still missed. And in our reproduction, it needs sophisticated situation to trigger that and we would like to add a unit test path for it

The submitted patch containing the unit test and the explanation of the unit test are in the git pull: https://github.com/apache/hbase/pull/5358