ref test-cases fetch re-resolves from the search spec, not the committed catalog

## Describe the bug
When a catalog already exists, `ref test-cases fetch` reruns the search rather than just fetching the datasets in the catalog.
This has implications for reproducibility.

## Expected behavior
If a catalog exists everyone uses the same datasets, unless a user runs with `--regen`.

The fetch process should use the instance ids from the catalog instead of the general search query.