Update XamlIslandsTest_StringExtensions to fix from Extensions namesp…

[michael-hawker]

…ace change

Related to #3743

Fixes

PR Type

What kind of change does this PR introduce?

What is the current behavior?

What is the new behavior?

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Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
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• Sample in sample app has been added / updated (for bug fixes / features)
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[ghost]

Thanks michael-hawker for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

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[Sergio0694]

Looks good, hopefully devs will not ever stumble into this themselves, as I'd imagine they'd all just be using the extension method syntax from the start anyway 😄