tests: ensure that extern "Rust" methods on C++ types work

[ahayzen-kdab]

This allows for developers and crates that are generating both C++ and Rust code to have a C++ method implemented in Rust without having to use a free method and passing through the C++ "this" as an argument.

[ahayzen-kdab]

While trying to simplify our free functions and wrapper methods going between generated C++ classes, Rust structs, and methods implemented in a C++ context in Rust. We found that it was possible to implement a Rust method onto a C++ type. As CXX only generates the implementation for it and we are generating the C++ class we can specify the right header match. Then this avoids us having to have a free function to proxy C++ calls to the Rust calls as we can then go directly between.

As this was slightly non-obvious we wanted to submit a test to ensure that the behaviour doesn't regress and to ensure that it is accepted behaviour that we can depend on.