Align testing docs with current test setup

[ekamran]

What?

Follow up to contributor documentation cleanup.

Updates the testing and contributor documentation so it matches the repository's current test setup.

Why?

The docs were referencing test structure and commands that do not match the current repository state. In particular:

• docs/TESTING.md referenced a tests/Unit/ suite that does not currently exist.
• The docs referenced outdated PHPUnit suite names.
• The docs used composer stan, while the repo currently exposes composer phpstan.

This can confuse first-time contributors and make setup feel broken even when the issue is only outdated documentation.

How?

• Updated docs/TESTING.md to reflect the current PHPUnit configuration.
• Replaced outdated suite examples with the current integration suite.
• Updated static analysis command references from composer stan to composer phpstan.
• Updated docs/DEVELOPER_GUIDE.md so contributors are directed to the existing tests/Integration/ suite instead of a non-existent tests/Unit/ directory.

Use of AI Tools

I used AI assistance to help inspect the repository, identify outdated documentation, and draft the documentation updates. I manually reviewed the repository structure, verified the current test configuration, and reviewed the final changes before submitting.

Testing Instructions

1. Open docs/TESTING.md and confirm the documented PHPUnit commands match phpunit.xml.dist.
2. Confirm docs/TESTING.md no longer claims the repository currently contains a tests/Unit/ suite.
3. Open docs/DEVELOPER_GUIDE.md and confirm the testing guidance points contributors to tests/Integration/.
4. Confirm the static analysis command documented in both files is composer phpstan.

Testing Instructions for Keyboard

1. Open the changed documentation files.
2. Navigate through the updated sections using keyboard-only navigation.
3. Confirm the content is readable and no UI interaction changes are involved.

Screenshots or screencast

Not applicable.

Before	After
Docs referenced a non-existent tests/Unit/ suite and outdated PHPUnit commands.	Docs now match the current integration-based test setup and commands.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ekamran <ekamran@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: justlevine <justlevine@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.44%. Comparing base (8322852) to head (df58292).
⚠️ Report is 71 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #393      +/-   ##
=============================================
+ Coverage      68.36%   68.44%   +0.08%     
- Complexity       813      846      +33     
=============================================
  Files             53       56       +3     
  Lines           3945     4095     +150     
=============================================
+ Hits            2697     2803     +106     
- Misses          1248     1292      +44     

Flag	Coverage Δ
unit	68.44% <ø> (+0.08%)	⬆️

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[jeffpaul]

Updates seem fine to me, will defer to @dkotter for final review/merge