steve-cmd fetch tests

[willkg]

Fetch does a bunch of things not covered by the current tests. This issue covers adding these tests:

1. verify fetch fails if there's no url
2. verify fetch fetches videos (use mocked youtube-dl)
3. verify fetch fetches videos and stomps on existing data with --force

Relatedly, should we adjust fetch? Is this ui helpful for the things we're asking it to do? Is "fetch" a good name?