Share caches between kernel test methods

Problem/Motivation

Kernel tests force a memory backend for cache bins.

If we use the database cache and share the bins between methods, this can re-use discovery and other expensive to build cache items.

In local profiling, it's saving about 300ms or so from each method of NodeTypeValidationTest but also causes at least one test failure.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

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