Promote unit testability via code best practices

Problem/Motivation

As pointed out in #3627430: [META] Adopt the Test Pyramid approach, we currently don't have clear guidelines on how to write code that's easy to unit test. A few ideas were demoed in this session but nothing has been officially agreed upon.

Proposed resolution

Provide best practices to:

• Introduce interfaces wherever a test double would be useful and appropriate.
  • Do not replace concrete classes with test doubles.
• Break code down into meaningful and cohesive units adhering to the SRP principle
• Isolate logic that reaches outside the PHP process (DB queries, HTTP requests) or interacts with third-party libraries behind thin adapter layers, such as the Repository pattern.
• Ban entity terminology from modules as much as possible, rely on domain-specific terminology instead.

Details TBD.

Remaining tasks

• Discuss, validate, and finalize the proposed solution
• Define the desired deliverable
• Produce the deliverable
• Review it

Introduced terminology

TBD

API changes

None foreseen

Data model changes

None foreseen

Release notes snippet

TBD