#2202185: Statically cache Role entities to speed up (User|UserSession)::hasPermission() wants to enable entity static caching for user roles. #1885830: Enable static caching for config entities wants to do it by default for all config entities. Both issues have run into the problem that the cache is broken as per this issue's title, since it is not yet used by any config entities in core.
Regardless of which, if any, core config entities we want to statically cache, it needs to be possible for contrib to enable static caching for their own entities, or turn it on for core ones.
Therefore, this issue is to add a test that is independent of core usages and fix the found bugs.