[12.x] Introduce WithCachedConfig testing trait

[cosmastech]

Building off of #57623

This change has two benefits:

1. We cache the Application@configurationIsCached() check, so this will benefit anyone running the app to reduce unnecessary repeated file_exists() for the cached config. This has been moved into its own PR: [12.x] Cache the result of configurationIsCached() #57665
2. Creates a testing trait for caching the configuration for all tests

On our pipeline, I observed the following:

• Original: 07.52.903
• WithCachedRoutes: 05:41.584
• WithCachedRoutes and WithCachedConfig: 04:28.769 🤯

[cosmastech]

Is there any concern here about someone storing an object in cache that can be modified between tests? We could serialize/unserialize it on each test. This would cost some CPU cycles for each test, but could insulate us from that problem. Not sure how often people are storing mutable objects in their configurations 🤔