private static byte[][] makeN(byte[] base, int n) {
    byte[][] ret = new byte[n][];
    for (int i = 0; i < n; i++) {
      ret[i] = Bytes.add(base, Bytes.toBytes(String.format("%04d", i)));
    }
    return ret;
  }
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