Currently we write Windows tests indirectly via the client wrapper APIs.

It would be nice to have a test helper API to write tests directly against the Windows C API, with many of the features we support e.g. in writing tests directly against the embedder API (using the EmbedderTestContext) or to a much more limited extent on macOS (using FlutterEngineTest).

This tracks:

1. creating such an API
2. writing the first test using that API and getting the infra in place to run it