To prep for making a combined command to run native tests across different platforms, rework `xctest`:
- Split analyze out into a new `xcode-analyze` command:
- Since the analyze step runs a new build over everything with different flags, this is only a small amount slower than the combined version
- This makes the logic easier to follow
- This allows us to meaningfully report skips, to better notice missing tests.
- Add the ability to target specific test bundles (RunnerTests or RunnerUITests)
To share code between the commands, this extracts a new `Xcode` helper class.
Part of https://github.com/flutter/flutter/issues/84392 and https://github.com/flutter/flutter/issues/86489
