patman: Add a test for the 'possible new uclass' check

patman: Add a test for the 'possible new uclass' check

It is quite likely that the number of U-Boot-specific tests in
checkpatch.pl will increase over time. We should have tests for these to
avoid undefined behaviour and bugs being introduced, which might cause
people to ignore the warnings.

Add a simple new class that can generate a patch with a single-line
addition in it. Use that to add a test for one of the checkpatch checks.

Signed-off-by: Simon Glass <sjg@chromium.org>