[HBASE-9703] Fix TestCompaction

Author: manukranthk

Summary: TestCompaction extends HBaseTestCase, but due to a change in HBaseTestCase TestCompaction was refereing to MiniDFSCluster variable which is no longer initialized. Reusing the MiniDFSCluster initialized by the HBaseTestCase fixes the test case.

Test Plan: Run TestCompaction

Reviewers: adela, aaiyer, rshroff

Reviewed By: rshroff

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D993497

Task ID: 2966736

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1528956 13f79535-47bb-0310-9956-ffa450edef68