Add testing guide

[samdark]

Closes #265.

Adds the testing guide section with pages for overview, environment setup, unit tests, functional tests, end-to-end tests, and static analysis with mutation testing.

Functional testing guidance uses PSR-7 requests as input and PSR-7 responses as output, without depending on a specific test framework.

[Copilot]

Pull request overview

Adds a new Testing section to the Yii3 definitive guide, covering the main testing levels and complementary quality tools to help keep applications and packages safe to change.

Changes:

• Introduces new testing guide pages: overview, environment setup, unit, functional, and end-to-end testing.
• Adds guidance on static analysis and mutation testing (Psalm/PHPStan + Infection).
• Updates the guide index to link to the new testing pages (replacing TODO entries).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/guide/testing/overview.md	Introduces testing levels and guidance on choosing the smallest effective test type.
src/guide/testing/environment-setup.md	Describes test environment isolation and state reset strategies.
src/guide/testing/unit.md	Adds unit testing principles and a PHPUnit example.
src/guide/testing/functional.md	Documents PSR-7/PSR-17 functional testing flow using Yiisoft\Yii\Http\Application.
src/guide/testing/end-to-end.md	Adds end-to-end testing guidance and environment considerations.
src/guide/testing/quality-tools.md	Adds static analysis and mutation testing guidance and when to run them.
src/guide/index.md	Updates the Testing section links to point to the new pages.