of: update kconfig unittest help

Add more information about the impact the of unittests have on the
live devicetree and why the tests should only be enabled for developer
kernels.

Add information about processing the test output such that the
results are more complete and comprehendable.

Signed-off-by: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20230213185702.395776-4-frowand.list@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>