private Table createTableAndLoadData(HMaster master, TableName tablename) throws Exception {
    return createTableAndLoadData(master, tablename, INITIAL_REGION_NUM, 1);
  }
This test tests 1, merging region not online; 2, merging same two regions; 3, merging unknown
regions. They are in one test case so that we don't have to create many tables, and these tests
are simple.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestRegionMergeTransactionOnCluster.java