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
Cleans up header order/grouping for consistency: associated header, C/C++ system/standard library headers, library headers, platform-specific #includes.
Adds <cstring> where strlen, memcpy are being used: there are a bunch of places we use them transitively.
Applies linter-required cleanups. Disables linter on one file due to included RapidJson header. See https://github.com/flutter/flutter/issues/65676
This patch does not cover flutter/shell/platform/darwin. There's a separate, slightly more intensive cleanup for those in progress.