[META] Adopt the Test Pyramid approach

The Drupal test suite has performance and maintainability issues that could be both addressed, together with consistency and readability, via an effort to make it adopt the Test Pyramid approach, as advocated for in this DC Rotterdam Core Conversation.

The Plan

• Improve entity unit testability
• Come up with best practices for Production code and Test code to promote unit testability
• Create an example/template module
• Make core modules adopt it without losing coverage in the process
• Update AI best practices

Proposed solution

A basic rule

Whenever writing a new test, a two-question rule of thumb should used:

1. Level: what’s the test scope? Unit, integration, or end-to-end
2. Type: what's the lightest available tool?
   • The default is a vanilla UnitTestCase test, possibly augmented via lightweight helper traits
   • Only move up the ladder if absolutely required