[test] Use the SDK provided in the CMake configuration.
[test] Use the SDK provided in the CMake configuration.

...rather than just asking xcrun to give us one. This is the one we
built for; we ought to be using it!

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