swift234

[build-script] Only build XCTest once

Prior to https://github.com/apple/swift-corelibs-xctest/pull/57 and
https://github.com/apple/swift-corelibs-xctest/pull/58, the
swift-corelibs-xctest build script did not provide a way an already
built XCTest.so to be tested or installed. As a result, the Swift
build script would build XCTest several times.

For example, the following invocation would build XCTest three separate
times:

```
$ utils/build-script --xctest --test -- --install-xctest
```

Modifications to the XCTest build script now allow a prebuilt XCTest to
be tested (via the `build_script.py test` subcommmand) and installed
(via the `build_script.py install` subcommand). Use these in the Swift
build script to build XCTest only once, then test and install the built
version.

Also, explicitly disable XCTest installation on OS X. XCTest's
`build_script.py` only supports non-Darwin platforms, so running
`utils/build-script --xctest -- --install-xctest` on OS X used to cause
the build script to fail because of how that script invokes `swiftc`.
It now fails with an explicit error indicating `--install-xctest` is
unsupported.