initial attempt to create case-by-case fixtures to address #193

These fixtures are generated for 14 specific cases for testing. We'd write tests for each of these cases -- for example, the bare requirements, a shapefile with WMS only, a scanned map with IIIF, etc. The idea being that we use the fixture in lieu of a layer from `selected.json` and keep the fixtures to the bare minimum without having to maintain real data for the fixtures.

The cases I've drawn up are based on what I've seen in the OGP network metadata and for how adopters might generate their metadata.