Write more tests

[marcorosa]

There are some tests that are missing:

• [ ] add_discovery not covered yet (whereas add_discoveries is covered)
• [ ] tests for embeddings
• [ ] test for update_similar_snippets
• [x] tests for #145 once it gets implemented
• [ ] test for check_repo with and without username/token
• [ ] tests for the UI clients
• [ ] ...

I will try to keep an updated list. Feel free to add missing tests by commenting this issue