mtd: mtd_subpagetest: use mtd_test helpers

Use mtdtest_scan_for_bad_eraseblocks() and mtdtest_erase_good_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>