The Metal context/surface code was only in the `flutter` namespace but should have been in `flutter::testing` for consistency with everything else in the `testing` directory.
Also squashes the declarations in the rest of that directory to match the style guide while I've got the macro still handy.
[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
Added a new `FlutterEngineAOTData` argument to `FlutterProjectArgs`. Embedders can instantiate and destroy this object via the new `FlutterEngineCreateAOTData` and `FlutterEngineCollectAOTData` methods provided.
If an embedder provides more than one source of AOT data to `FlutterEngineInitialize` or `FlutterEngineRun` (e.g. snapshots as well as `FlutterEngineAOTData`), the engine will error out.
Resolves: https://github.com/flutter/flutter/issues/50778
