tests/subsys/settings/nffs: fix not initialized variables issues

A few settings module variables were not initialized before used.
Normally these variable are initialized in the back-end
initialization call which couldn't be done in affected unit tests.

This path initialize these variable via assignments in test code.

fixes #19722

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>