Adapt the tests to use the standard Redis docker image where possible,
instead of using the Redis Stack image in all places. This way we can
run the CI, at least in theory, against different versions of Redis and
Redis Stack dockers.

Run the test cluster based on the default Redis docker image, i.e.
without Stack support. This is needed in order to make the CI pass until
Stack gets a new release. What is needed is to mark all Stack related
tests accordingly, and then they don't get executed in the cluster
tests (they are already filtered out in `invoke cluster-tests`).

Take the opportunity to remove some empty tests and to fix the naming in
two tests.