of: prepare to add processing of EXPECT_NOT to of_unittest_expect

of: prepare to add processing of EXPECT_NOT to of_unittest_expect

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 1 of 2, renaming existing variables in anticipation
of the changes needed to process EXPECT_NOT, so that commit 2 of 2
will be smaller and easier to read.  This commit is not intended
to make any functional changes.

Signed-off-by: Frank Rowand <frowand.list@gmail.com>
Link: https://lore.kernel.org/r/20230213185702.395776-2-frowand.list@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>