tests/storage/flash_map: Support for no flash_area_open

The commit removes flash_area_open usage from tests, where it is not tested
and only used to obtain area.
The commit adds tests for flash_area_device_is_ready.
The commit adds test for flash_area_sectors, which is equivalent
of flash_area_get_sectors but takes flash area object instead of flash
area index.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>