add transform expressions tests (https://github.com/mapnik/mapnik/pull/141\ 6) (ref #3569)
add transform expressions tests (https://github.com/mapnik/mapnik/pull/141\
6) (ref #3569)

lightmare/mapnik,mapnik/mapnik,lightmare/mapnik,lightmare/mapnik,naturalatlas/mapnik,mapnik/mapnik,mapnik/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,tomhughes/mapnik,lightmare/mapnik,tomhughes/mapnik,tomhughes/mapnik,mapnik/mapnik,naturalatlas/mapnik,tomhughes/mapnik