mtd: mtd_stresstest: use mtd_test helpers

Use mtdtest_read(), mtdtest_write(), mtdtest_erase_eraseblock(), and
mtdtest_scan_for_bad_eraseblocks() in mtd_test helpers.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Vikram Narayanan <vikram186@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>