`__CommandLineArguments_v0` should default to JSON schema v0 rather than snapshot JSON

[grynspan]

Per this paragraph in #479:

If --event-stream-output is specified but --event-stream-version is not, the format currently used is based on direct JSON encodings of the internal Swift structures used by Swift Testing. This format is necessary to support Xcode 16 Beta 1. In the future, the default value of this argument will be assumed to be 0 instead (i.e. the JSON schema will match what we are proposing here.)

We should switch the default behaviour. To continue supporting Xcode 16 betas, we can continue to emit the older snapshot format if the requested version is -1 instead (which is not a supported version and would only be used until Xcode can adopt the v0 JSON schema.)

[grynspan]

@swift-ci please test