Simplify ANIM_unit_mapping_get_factor() by returning early and reducing cognitive complexity. This will make it significantly simpler to expand it for other RNA property (sub)types. Also I added a unit test for rotation conversion between radians and degrees. No functional changes. This is intended as a cleanup for !143880. Taken over from #144140 Co-authored-by: Sybren A. Stüvel <sybren@blender.org> Pull Request: https://projects.blender.org/blender/blender/pulls/144427