Test coverage of mapnik::vector_tile_impl::tile_layer

[springmeyer]

While trying to fix a bug I think is inside the new tile_layer class I noticed the tests for this class are commented: https://github.com/mapbox/mapnik-vector-tile/blob/master/test/unit/tile_impl/tile.cpp#L195-L338

Looks like they were disabled @flippmoke in a9e306b#diff-500e66444d0e17c2ca2cf308f8b81497L193

[flippmoke]

These where old tests prior to the changes in the layer class, I have removed them.