mtd: mtd_nandecctest: add no corruption test

This adds no corruptin test case listed below:

Prepare data block and ECC data with no corruption, and verify that
the data block is preserved by __nand_correct_data()

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>