Tests: Modernize PHPUnit tests in tests/phpunit/tests/wp-includes/functions/ - Trac #53010, #65208

[pbearne]

This patch should be applied after https://core.trac.wordpress.org/ticket/65252

Modernizes and reorganizes the PHPUnit test suite for wp-includes/functions.php located in tests/phpunit/tests/wp-includes/functions/ to improve maintainability, automated test reliability, and compatibility with modern PHPUnit versions (10, 11, and 12), following Trac tickets #53010, #65208, and #62004:

• Directory & File Restructuring: Moved tests from tests/phpunit/tests/functions/ to tests/phpunit/tests/wp-includes/functions/ using git mv to preserve git history.
• Granular Single-Unit Test Classes: Split multi-unit test files (anonymization.php, cleanDirsizeCache.php, maybeSerialize.php, referer.php, underscoreReturn.php, wpListUtil.php, xmlrpc.php) into dedicated, single-unit test classes.
• Naming Standards: Renamed all 128 test files and test classes to PascalCase ending with the Test suffix, ensuring an exact case-sensitive match between class and file names.
• PSR-4 Namespacing: Applied the WordPress\Tests\WP_Includes\Functions namespace across all test classes with explicit use imports for WP_UnitTestCase and other test dependencies.
• Class-Level @covers Annotations: Consolidated precise @covers tags into opening class docblocks and removed method-level @covers tags.
• Assertion Strictness & Isolation: Validated all 1,420 tests passing in standard execution order and across multiple randomized seeds (--order-by=random --random-order-seed=1 / 2) to ensure complete test isolation and zero state pollution.

Trac ticket: https://core.trac.wordpress.org/ticket/53010
See also: https://core.trac.wordpress.org/ticket/65208, https://core.trac.wordpress.org/ticket/62004

Use of AI Tools

AI assistance: Yes
Tool(s): Junie
Model(s): gemini-3.7-flash
Used for: Automated file splitting, PSR-4 namespacing, @covers docblock consolidation, and test execution validation.

[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.

Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @pbearne@git.wordpress.org.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

Core Committers: Use this line as a base for the props when committing in SVN:

Props pbearne.

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

[pbearne]

as we are deleting/moving files this will some help to merge