tests: drivers: flash: common: Fix page size when no erase is required

When no flash device requires erase, this test does not retrieve
flash page size with flash_get_page_info_by_offs(), but instead
it takes an arbitrary page size based on the test area length.
Since the test_flash_copy routine needs to use two pages, the test
area needs to be split into at least two parts. Correct the related
code and add a check if test_flash_copy requirements are met.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>