of: unittest: taint the kernel when of unittest runs

Make OF unittest trigger the new TAINT_TEST taint when OF unittest runs.
Due to OF unittest not being intended to run on production systems, and
potentially causing problems (or security issues like leaking kernel
addresses), the kernel's state should not be considered safe for
production use after OF unittest runs.

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Link: https://lore.kernel.org/r/20220823200152.3465751-1-frowand.list@gmail.com


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