disable table in HBaseTestingUtility.truncateTable

Old truncate table expected table to be enabled and removed rows by scanning over them.
In HBASE-13764, the function was changed to make it consistent with how shell worked, so new one used admin.truncateTable.
Since HBTU.truncateTable expects table to be enabled (backward compat) whereas admin.truncateTable expects table to be disabled, we should add a disable table there.