Add invenio-cache (Redis) fixture

[slint]

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

• Clear all keys with pefix CACHE_KEY_REFIX
• 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.

[ppanero]

See https://github.com/inveniosoftware/invenio-records-resources/pull/191/files#diff-0ba313cb2999f359508cf4f109393ff1549358fb857cd9c3620266ad7994a627R71 for reference

[ppanero]

An ad-hoc fixture has been placed in invenio-record-resources, invenio-vocabularies and invenio-rdm-records, remove from there when fixing this issue:

@pytest.fixture()
def cache():
    """Empty cache."""
    try:
        current_cache.clear()
        yield current_cache
    finally:
        current_cache.clear()