Cache Self and Director tests should test both cache hit and cache miss scenarios

[jhiemstrawisc]

We recently ran into an operational bug where object fetches through caches were failing, but only for cache hits. We didn't detect this in our monitoring because the current director/self tests for caches only ever test the cold cache case.

Let's extend the tests to fetch the same test object twice -- the first covers the cache miss case, and the second covers the cache hit case.