wish: Windows test where DevDeviceId::get() returns Ok(None)

I want a test that shows that DevDeviceId::get() on Windows returns Ok(None) if there's no device id in the registry. Of course on any regular dev machine that won't be the case if you have run the test before, or if you have various Microsoft developer tools installed.

On Unix we could fake it by setting HOME to an empty temporary directory during the test.

On Windows we can't really fake an empty registry.

One approach is to use a mock: instead of using the functions from the winreg crate directly, we make a WindowsRegistry trait and two implementation structs. One that just calls the winreg functions and a MockRegistry struct that either returns canned responses or uses an in-memory database, and run the tests against a fake registry.