mtd: tests: annotate as DANGEROUS in Kconfig

The tests may erase mtd devices, so annotate them as suggested per
coding style and add a paragraph to the help text as well.

Artem: amended the help test a bit.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@intel.com>