HBASE-8611. Improve test coverage in pkg org.apache.hadoop.hbase.mapred

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1537567 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HBASE-8584-trunk-gd