patman: Add a maintainer test feature to MakeCcFile()

Allow the add_maintainers parameter to be a list of maintainers, thus
allowing us to simulate calling the script in tests without actually
needing it to work.

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