tests/subsys/settings/functional: ensure settings area is cleared

The test assumes that the settings area is empty.  This happens
naturally when the area is in the SOC flash and the chip flash is
entirely erased when the test is programmed.  The test will fail if the
programmed board is reset and the test run again.

The test cannot pass by reprogramming when the storage area is in an
external flash device.

Make things repeatable for FCB by erasing the storage partition before
running the test.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>