tests/subsys/fs/nvs: Fix flakiness due to not init'ed data

This test has been seen failing at random (though not often).
A valgrind check showed the nvs_ate structures were being
CRC'ed with not-initialized data, and checked later (assuming
the CRC would be different than a constant).
This maybe have been the cause of the test failures, so let's
initialize the whole structure to prevent the CRC value
from depending on random memory/stack content.

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