Add invenio-cache (Redis) fixture

It would be useful to have `invenio-cache` (or Redis) fixtures to be able to:

- [x] Clear all keys with pefix [`CACHE_KEY_REFIX`](https://github.com/inveniosoftware/invenio-cache/blob/2f0e1c8f9b1cf2b2dfb338d82b77f6287a367a74/invenio_cache/config.py#L19)
- [x] Clear the entire Redis instance (e.g. with `FLUSHALL`)

Another interesting alternative is specifying a custom `CACHE_KEY_PREFIX` for testing (e.g. `'tests::cache::'`) and clear only this one.

Start by locating the existing pytest-invenio fixture definitions and checking how invenio-cache or Redis is configured in tests. Compare the proposed key-prefix cleanup with FLUSHALL, then add a fixture whose behavior is covered for clearing the intended cache state.

Written by the indexing model from the issue text.