Skip individual tests upon Dalli::RingError

Unlike the outer `rescue`, this one is much more precise about what we
want to handle: a connection failure (`Dalli::RingError`) is not
relevant to what we're testing here. But other Dalli errors may well be
indicating an actual problem.
Merge pull request #15345 from matthewd/skip-dalli-errors

Skip individual tests upon Dalli::RingError