[build-presets] Add corelibs-xctest CI preset
[build-presets] Add corelibs-xctest CI preset

This preset will be used to test swift-corelibs-xctest on Apple CI.
It's also pretty handy for contributors to swift-corelibs-xctest.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift