Create unit test comparing datasette metadata inputs/outputs

[katie-lamb]

Currently the integration test for the Datasette metadata isn't that exhaustive.

Create a unit test that has its own self-contained collection of minimal metadata, which we can run through the metadata_to_yml script and parse using datasette.utiles.parse_metadata() and compare the inputs/outputs exhaustively.

Separately, there might be updates to Datasette metadata validation in future versions of Datasette. Some Datasette issues to track:
Validate metadata.json on startup

Verification checks for metadata.json on startup

Datasette 1.0