Support PHPUnit TestSuite factories when constructors are private

[chubes4]

Summary

The generated wordpress.phpunit Playground harness crashes during load_tests with PHPUnit 10+ because it directly constructs PHPUnit\Framework\TestSuite, whose constructor is private in those versions.

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

Reproduction

A Homeboy WordPress release gate using WP Codebox 0.12.14 installed WordPress and dependencies and discovered 46 *Test.php files, then failed before any component test ran:

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

The defect remains on current main at both generated harness sites:

• packages/runtime-playground/src/phpunit-command-handlers.ts:1273 (phpunitRunCode)
• packages/runtime-playground/src/phpunit-command-handlers.ts:1592 (corePhpunitRunCode)

Both emit new PHPUnit\Framework\TestSuite(...).

Compatibility approach

Use the public factory when available, preserving PHPUnit 9 compatibility:

$suite = method_exists(PHPUnit\Framework\TestSuite::class, 'empty')
    ? PHPUnit\Framework\TestSuite::empty('WP Codebox PHPUnit Tests')
    : new PHPUnit\Framework\TestSuite('WP Codebox PHPUnit Tests');

PHPUnit 10 through current PHPUnit expose TestSuite::empty() and keep the constructor private. PHPUnit 9 has a public constructor and no empty() factory.

The discovered-class path also currently calls $suite->addTestSuite($class_name). PHPUnit 10+ requires a ReflectionClass; PHPUnit 9 accepts either a class name or ReflectionClass, so passing the existing $ref is compatible across both API generations.

Acceptance criteria

• Update plugin and WordPress-core generated harnesses.
• Add generated-code assertions covering the public factory fallback.
• Add focused execution coverage with a private-constructor TestSuite double proving discovered test files reach execution.
• Preserve distinct STAGE_FAIL / STAGE_FATAL infrastructure reporting versus test assertion failures.

[chubes4]

Superseded by #1926 and fixed by merged PR #1927.