swift234

[build-script] Build & test XCTest (on OS X, too)

Allow swift-corelibs-xctest to be built and tested on Linux and OS X,
via `utils/build-script --xctest --test`.

On OS X, XCTest is built via `xcodebuild`, which has been possible since
https://github.com/apple/swift-corelibs-xctest/pull/47. It's tested via
the "SwiftXCTestFunctionalTests" Xcode target. Keep in mind that
`xcodebuild` must be configured on the host machine to use a Swift
toolchain that can build XCTest--as of
https://github.com/apple/swift-corelibs-xctest/pull/48, that would be
`swift-DEVELOPMENT-SNAPSHOT-2016-02-08` or later.

On Linux, XCTest is built and tested via the project's
`build_script.py`, which has been possible since
https://github.com/apple/swift-corelibs-xctest/pull/46.