Make null_store the default cache store in test environment config

[mcnelson]

Summary

It's surprising to me that by default, low-level caching is functional in the test environment. This simply adds config.cache_store = :null_store in config/environments/test.rb.

The existing defaults in test.rb as well as the Rails Guides kinda have you believe the null_store is the default.

Really was on the fence about opening this as it may not be a substantial enough change to warrant dev resources. But since Rails has such a great reputation of providing sane defaults, I felt like this would be worth it.

Other Information

I perused the tests and wasn't sure where best to add anything. railties/test/application/generators_test.rb?

Appropriate to update CHANGELOG?

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[georgeclaghorn]

I’ve stubbed my toe on this before. 👍

Nothing changes for existing apps unless they opt into the new default via rails app:update, so I don’t think a changelog entry is necessary. Thanks!

[eileencodes]

@mcnelson can you open a new PR with a changelog? It's important to mention this change in behavior.

[mcnelson]

Done: #33779