drm/todo: Add entry for using kunit in the subsystem

drm/todo: Add entry for using kunit in the subsystem

The Kernel Unit Testing (KUnit) framework provides a common framework for
unit tests within the Linux kernel. Having a test suite would allow to
identify regressions earlier.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220504080212.713275-1-javierm@redhat.com