Refactor serializer tests to shared base with DOM-specific subclasses

[Copilot]

PR #257’s serializer coverage for PHP 8.4 Dom\HTMLDocument duplicated nearly the entire legacy OutputRulesTest suite. This refactor keeps equivalent coverage while centralizing shared assertions and isolating only runtime-specific behavior.

• Shared test consolidation

  • Introduced AbstractOutputRulesTest with common OutputRules serializer assertions.
  • Moved duplicated setup/helpers and shared test cases into the base class.

• DOM-specific specialization

  • OutputRulesTest now acts as the legacy DOMDocument subclass.
  • OutputRulesNewDomTest now acts as the PHP 8.4 Dom\HTMLDocument subclass.
  • Kept only behavior deltas in subclasses (e.g., namespace serialization shape, PI creation path, invalid-raw-content skip in new DOM).

• Compatibility helpers

  • Added overridable node-factory helpers in the base class (createCommentNode, createTextNode, createElementWithText) so each DOM implementation can construct nodes safely without duplicating full test methods.

abstract class AbstractOutputRulesTest extends TestCase
{
    abstract protected function loadHTML($html);

    protected function createElementWithText($dom, $name, $value)
    {
        return $dom->createElement($name, $value); // legacy DOM
    }
}

final class OutputRulesNewDomTest extends AbstractOutputRulesTest
{
    protected function loadHTML($html)
    {
        return HTMLDocument::createFromString($html, $options);
    }

    protected function createElementWithText($dom, $name, $value)
    {
        $el = $dom->createElement($name);
        $el->textContent = $value;
        return $el;
    }
}