patman: Add a functional test

The existing test (patman --test) only covers basic checkpatch output.
We have had some problems with unicode processing and could use test
coverage for the various tags patman supports.

Add a new functional test which runs most of the patman flow on a few
test commits and checks that the results are correct.

See the documentation in the test for a description of what it does.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>