tests/subsys/storage/flash_map: use auto-generated fa ID

After addition of flash_map auto-generation it is needed to
use generated flas_area name instead of hard-code.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>