Creates a new `native-test` command that will be used to run native unit and UI/integration tests for all platforms over time. This replaces both `xctest` and `java-test`.
For CI we can continue to run each platform separately for clarity, but the combined command makes it easier to use (and remember how to use) for local development, as well as avoiding the need to introduce several new commands for desktop testing as support for that is added to the tool.
Fixes https://github.com/flutter/flutter/issues/84392
Fixes https://github.com/flutter/flutter/issues/86489
