tests/subsys/settings/fcb_int: add storage preparation

The test assumes that storage is clear before it runs. This
might be not true as lot of devices are only partially erased by
'west flash'.

Patch introduce procedure for testing whether test runs the first
time and clear storage if so.

The procedure uses mark which is stored inside SoC embedded program
flash. It will not work one devices on which read/write to it is
impossible.

fixes #16463

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