scripts/dtc/of_unittest_expect processes EXPECT messages that
document expected kernel messages triggered by unittest.  Add
processing of EXPECT_NOT messages that document kernel messages
triggered by unittest that are not expected.

This is commit 2 of 2, implementing the processing of EXPECT_NOT
messages.
Signed-off-by: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20230213185702.395776-3-frowand.list@gmail.comSigned-off-by: Rob Herring <robh@kernel.org>