patman: Fix tests if add_maintainers is set to False

If add_maintainers is set to False in the user's ~/.patman config, it will
cause the custom_get_maintainer_script to fail since that test expects
maintainers to be added. Set add_maintainer to True in the .patman config
to prevent this.

Fixes: 8c042fb7 ("patman: add '--get-maintainer-script' argument")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>