Add/adopt high level cache tests and run them against each backend

We currently have NULL, Install and Database cache backends, #1596472: Replace hard coded static cache of entities with cache backends will add a fourth if it goes in.

While the NULL and Install backends would fail a comprehensive test since by design they don't store anything, we might be able to make some generic test stubs then run those against each one. If the Array backend gets committed, then that should behave about the same as the database backend so could definitely share some test infrastructure.