wanted: test that components handle known and unknown fields

[dustmop]

By modifying the data returned by tests/mock_loader.py, we should be able to show that constructed dataset components, like structure and meta, will be assigned fields from their list of known fields. See their initializers in qri/dataset.py. We should have tests that verify they get this state, show it when stringified, and will not have any unknown fields assigned to them.