tests/drivers/flash common: Allow on nrf52_bsim

903a7943
seems to have attempted to limit in how
many platforms this test runs due to issues in some
platforms. But how it was done (using platform allow apart
from integration platform) it prevents it running on
other platforms in which it works.
Add the nrf52_bsim target to this allow list so the
test can be run on it too.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>