Fix inconsistent specs around caching

`#concern_reload` would execute the Rails `caches_page` and
`caches_action` methods multiple times resulting in multiple callbacks
being associated with the controller. This lead to inconsistencies.
Instead `config.action_controller.perform_caching` is now set to true by
default. With this in place we no longer need to reopen the class in
order to test caching.

Refactor `action_caching` spec to test via spying rather than testing
caching directly. Now that we are no longer actually caching items, we
no longer need to clear the cache via
`ActionController::Base.cache_store.clear`.