Java, Swift, JavaScript, Go, Dart, Csharp have at least a few tests for the runtime APIs that directly make calls using test rigs written in the target language. Some are in api or testing dirs but others are in main target src dir.

I wonder when all of those were actually run! haha.

We should make these "first class" entities like our runtime-testing dir / infrastructure. Should set it up so they are run as part of CI or build.