Support PHPUnit versions with private TestSuite constructors

[chubes4]

Problem

The generated wordpress.phpunit workload directly instantiates PHPUnit\Framework\TestSuite. PHPUnit versions where TestSuite::__construct() is private crash during load_tests before any component tests execute.

Reproduction

A Homeboy release test for Data Machine Events using WP Codebox 0.12.14:

STAGE_FATAL:load_tests:Uncaught Error: Call to private PHPUnit\Framework\TestSuite::__construct() from global scope in /internal/eval.php:1096

The runtime successfully installs WordPress, activates dependencies, and discovers 46 test files. The failure is in generated WP Codebox harness code. Current origin/main still directly constructs TestSuite in both plugin and core harness generators.

Downstream tracking: Extra-Chill/homeboy-extensions#2258.

Expected

Construct suites through a PHPUnit-version-compatible public API so discovered tests reach execution across supported old and new PHPUnit versions.

Acceptance criteria

• Replace direct TestSuite construction in every generated PHPUnit harness path.
• Preserve compatibility with supported older PHPUnit versions.
• Add regression coverage for a PHPUnit version with a private constructor.
• Preserve stage-specific infrastructure failure reporting.
• Verify discovered *Test.php files reach execution.