Test JSON paths

[jpmckinney]

We should add a test to check that the argument to the get_values function is a valid OCDS JSON path. For example, in dataset/distribution/main_procurement_category.py, we'd test that tender.mainProcurementCategory is a JSON path that actually occurs in OCDS data.

https://ocdskit.readthedocs.io/en/latest/api/schema.html#ocdskit.schema.get_schema_fields can yield all JSON paths from the OCDS schema.