Add test component

[kennykerr]

This adds a WinRT test component (DLL) written in Rust. This ensures we test activation fallback logic.

[rylev]

Looks good in general! Just a few small comments 😊

[rylev]

This unfortunately puts a dependency on midlrt.exe to build windows-rs now. Two concerns:

• This would be nice if it was documented somewhere
• I only have access from midlrt.exe from the VS command prompt, but I normally build from powershell. How can I make sure that midlrt.exe is available from powershell?

[kennykerr]

Yes, I don't like this either. It was quite a slog getting this to work at all. We should probably document some of the pain points around getting this to work and see what we can improve (both with the midlrt dependency issues and the cargo target dependency issues).

Regarding midlrt, the use of this tool is temporary until I'm finished with my winmd generator at which point that will simply be yanked out. If there's a way to make this test not run by default with cargo test --all, or some other solution, that would be great.

[rylev]

I think we can find a solution. For example, we can feature gate it and then run the tests with that feature in CI.