Adapt `TestForeign.test_heifsave` test (#2402)
Adapt `TestForeign.test_heifsave` test (#2402)

- `heifsave` defaults to AV1 compression for .avif suffix since commit 8c44863.
- Test lossless compression mode.
jcupitt/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips