[Bugfix] Include Microsoft.XamlIslands in UnitTest.XamlIslands.UWPApp

[Rosuavio]

For some reason this fixes the build errors, it shouldint as its already a transitive dep.

Fixes #3781

Fixes build issue with UnitTest.XamlIslands.UWPApp

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

What is the new behavior?

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• New major technical changes in the toolkit have or will be added to the Wiki e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

[ghost]

Thanks RosarioPulella 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 🙌

[michael-hawker]

LGTM, thanks!