add tests for tiff pyr save region-shrink flag
add tests for tiff pyr save region-shrink flag

we were testing the flag before, but not that the result was correct

see https://github.com/libvips/libvips/issues/1875

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