mtd: tests: introduce helper functions

This introduces the helper functions which can be used by several
mtd/tests modules.

The following three functions are used all over the test modules.

- mtdtest_erase_eraseblock()
- mtdtest_scan_for_bad_eraseblocks()
- mtdtest_erase_good_eraseblocks()

The following are wrapper functions for mtd_read() and mtd_write()
which can simplify the return value check.

- mtdtest_read()
- mtdtest_write()

All helpers are put into a single .c file and it will be linked to
every test module later.  The code will actually be copied to every
test module, but it is fine for our small test infrastructure.

[dwmw2: merge later 'return -EIO when mtdtest_read() failed' fix]

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>