Add tests for filtering and sorting.
Add tests for filtering and sorting.

Add django unit tests to ensure the order and filtering of layers works
properly.

raster-foundry/raster-foundry,kdeloach/raster-foundry,kdeloach/raster-foundry,kdeloach/raster-foundry,azavea/raster-foundry,kdeloach/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,aaronxsu/raster-foundry,azavea/raster-foundry,kdeloach/raster-foundry,raster-foundry/raster-foundry,azavea/raster-foundry,raster-foundry/raster-foundry,aaronxsu/raster-foundry