Fix milvus and faiss tests not running

[tstadel]

Related Issues

• fixes tests not run in https://github.com/deepset-ai/haystack/pull/3252/

Proposed Changes:

• better inferencing of required document store
• fixed an issue with `test_faiss_and_milvus.py::test_cosine_similarity

How did you test it?

• well actually run them finally

Notes for the reviewer

this is a follow-up PR to https://github.com/deepset-ai/haystack/pull/3252/

Checklist

• [x] I have read the contributors guidelines and the code of conduct
• [ ] I have updated the related issue with new insights and changes
• [ ] I added tests that demonstrate the correct behavior of the change
• [x] I've used the conventional commit convention for my PR title
• [x] I documented my code
• [ ] I ran pre-commit hooks and fixed any issue