Port tests to .NET

We have a few test suites with a lot of valuable tests, but which don't run for .NET, only legacy Xamarin:

* [ ] [mtouch](https://github.com/xamarin/xamarin-macios/tree/main/tests/mtouch)
* [ ] #26148
* [x] [mmptest](https://github.com/xamarin/xamarin-macios/tree/main/tests/mmptest)
* [x] https://github.com/dotnet/macios/pull/26146
* [x] [msbuild](https://github.com/xamarin/xamarin-macios/tree/main/tests/msbuild) to some extent.
* [x] [generator](https://github.com/xamarin/xamarin-macios/tree/main/tests/generator) to some extent.

These tests should be ported somehow to run for .NET.

Part of the design for https://github.com/dotnet/macios/issues/17693 is to make it easier to port mtouch/mmp tests, so that needs to happen first.