[SwiftBindings] Setup xunit testing framework

[kotlarmilos]

Description

This PR introduces the following test improvements:

• Unit tests: Tests for components of the projection tooling.
• Integration tests: Tests that compile Swift code and invoke the projection tooling to generate bindings on macOS.
• Framework tests (TBD): Tests that consume bindings as a NuGet package, build tests/apps, and run on target platforms.

This change improves debugging by referencing generated bindings within the integration tests. Additionally, it removes need for reflection in tests. On non-macOS platforms, only the tooling is built, and tests are skipped.

The objective is to set up the pipeline for StoreKit2 and SwiftUI, and begin tracking bindings coverage using the projection tooling, which will be introduced in a follow-up PR.

[stephen-hawley]

I'd like to see this branch get migrated to use TypeMetadata instead of void * and be ready to use the upcoming metadata access method in TypeMetadata. Like what I'm seeing.