split heifload / heifsave tests
split heifload / heifsave tests

to allow building with the de265 decoder, but not the GPL ffmpeg x265 encoder

jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips