class in `test_responsecache.py` has the wrong name (`DeferredCacheTestCase` instead of `ResponseCacheTestCase`)

[ShadowJonathan]

This is my own fault, but i'm putting it here because i noticed it while working on something else.

The file under tests/util/caches/test_responsecache.py uses DeferredCacheTestCase, test_deferred_cache.py also uses this classname, so it has to be ResponseCacheTestCase instead.

[ShadowJonathan]

(@ who triages this, could you assign me to the issue?)