HBASE-8553. improve unit-test coverage of package org.apache.hadoop.hbase.mapreduce.hadoopbackport. Contributed by Ivan A. Veselovsky.

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1536502 13f79535-47bb-0310-9956-ffa450edef68