Internal: More reliable internal testing helper implementation

[caendesilva]

Fixes an issue where the old implementation does not work reliably on macOS when running the full suite.

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Explanation of Changes:

1. Using XPath: XPath is a powerful language for querying XML documents (and HTML, which is XML-like). It's much more efficient and reliable for complex queries than manual traversal.

2. DOMXPath Object: We create a DOMXPath object using the existing $this->document object, which is already a parsed DOM. This avoids re-parsing the HTML.

3. Robust Class Matching: The XPath expression //*[contains(concat(' ', normalize-space(@class), ' '), ' $class ')] is a standard and robust way to find elements with a specific class, even if they have multiple classes. It handles leading/trailing spaces correctly.

4. Iterating DOMNodeList: The query() method returns a DOMNodeList. We iterate over this list and add the matching elements to our collection.

5. Type Safety: The added if ($node instanceof DOMElement) check ensures type safety and prevents unexpected behavior if non-element nodes are encountered in the DOMNodeList.

6. parseNodeRecursive: We reuse your existing parseNodeRecursive method to convert the DOMElement objects into TestableHtmlElement objects for consistency.

Why this is better:

• Reliability: XPath is standardized and less prone to platform-specific quirks.
• Efficiency: XPath is generally faster than manual DOM traversal.
• Maintainability: The code is simpler and easier to understand.
• Robustness: The class matching logic is more robust.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e040ace) to head (673573c).
Report is 13 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master     #2057   +/-   ##
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  Coverage     100.00%   100.00%           
  Complexity      1766      1766           
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  Files            182       182           
  Lines           4758      4758           
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  Hits            4758      4758           

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