Add opportunity to use map in widget tests

[ManoyloK]

Hey! I needed to use the map in widget tests. But as it uses platform views those are not working properly during widget tests. So I made some classes visible and added a method to provide fake MethodChannelMaplibreGl to MapLibreGlPlatform.

Usage

class FakeMethodChannelMaplibreGl implements MethodChannelMaplibreGl {
...
}

 MapLibreGlPlatform.instance = FakeMethodChannelMaplibreGl();

[JulianBissekkou]

Thanks! @ManoyloK