tests: subsys: storage: flash_map: Extend timeout storage.flash_map.mpu

The test `storage.flash_map.mpu` for `nrf52840dk/nrf52840` fails
althought the output is correct. Time to test execution is not enough.
This change extend test execution timeout.

Signed-off-by: Katarzyna Giądła <katarzyna.giadla@nordicsemi.no>