Add loader snapshot tests

[ai]

In #10 we just have smoke-test (that functions do not fail) because real feeds updates every day.

But some feeds are dead (author do not add new posts) and we can use them in snapshot tests.

• [ ] Add marker to loader-tests/feeds.yml that it is dead feed.
• [ ] Add a way to generate feed/posts snapshots for that feeds.
• [ ] Add snapshot check to integration script.
• [ ] Add docs to loader-tests/ docs.