mtd: tests: Add mtd_erasepart test file.

MTD tests run only on erased partitions. This module takes care of erasing
the partition before running any test. This test is especially helpful
when the MTD tests are to be run in automation one after the other.

Change-Id: I6a2566fce662bf132ff4a73ae2417807f1d17810
Signed-off-by: Murali Nalajala <mnalajal@qualcomm.com>