More responsive and detailed tests

[GretaCB]

This ticket serves as a reminder for the future...

Per chat with @rclark , tests should be a bit more verbose. This will come in handy when gut-checking major changes to the metadata object, and will be relevant in the coming months when we do a major overhaul of the metadata json.

• Add more detailed assertions for info returned and for expected mapnikXML generated.
• Remove the hardcoded fixtures, and create fixtures on the fly based on metadata returned from mapnik-omnivore.