Description

Migrate the Controls.Xaml.UnitTests project from NUnit to xUnit as part of the repository-wide effort to standardize on xUnit for unit testing. This will ensure consistency, reduce maintenance complexity, and align with the overall migration tracked in #29977.

Tasks

• Replace all NUnit dependencies and test attributes with xUnit equivalents in the Controls.Xaml.UnitTests project.
• Update assertions and test fixtures as needed for xUnit compatibility.
• Remove NUnit references and obsolete code from the project.
• Ensure all tests run and pass after migration.
• Verify CI/CD integration for the migrated tests.

Acceptance Criteria

• All tests in Controls.Xaml.UnitTests are running on xUnit with no failures.
• No remaining NUnit dependencies in the project.
• Documentation is updated if required.

References

Parent Epic: #29977

xUnit docs: https://xunit.net/
NUnit docs: https://docs.nunit.org/

Do not migrate tests in the ./src/Compatibility folders.